The Future of Terrorist Use of Improvised Explosive Devices: Getting in Front of an Evolving Threat

Abstract: Because of their outsized impact—as well as the wide availability of device components—violent extremists in the United States consider improvised explosive devices a valuable part of their arsenal. They are easy to make, difficult to combat, and cause significant harm and disruption. Yet this threat is not static. Violent extremists continue to innovate, drawing on emerging technologies and creative problem solving. The onus of initiative requires that the counterterrorism mission community looks over the horizon to identify emerging threats. In support of this need, the authors offer a forward-leaning taxonomy of emerging threats related to terrorist use of IEDs in the United States and outline its key implications for policy, practice, and applied study.

On April 15, 2013, two improvised explosive devices (IEDs) were placed roughly 200 yards apart on Boylston Street in Boston, Massachusetts, and detonated near the final stretch of the Boston Marathon.1 The joint explosions killed three people—eight-year-old Martin Richard, 23-year-old Lu Lingzi, and 29-year-old Krystle Campbell—and injured more than 250 others.2

Investigators concluded that the attackers were motivated by extremist beliefs, though “not connected to any known terrorist groups.”3 The perpetrators responsible for the attack, 26-year-old Tamerlan Tsarnaev and 19-year-old Dzhokhar Tsarnaev, had learned how to construct the explosive devices from a popular article published in Inspire magazine, an English-language online publication produced by al-Qa`ida in the Arabian Peninsula.4 The improvised devices used in the attack were two six-quart pressure cookers packed with low explosives, ball bearings, nails, and other metal used as shrapnel.5 The explosions most likely came from up to 20 pounds of powder from fireworks and/or similar pyrotechnic materials.6 The attackers detonated each bomb remotely by sending a signal to a receiver on each device, which used power from battery packs to light Christmas tree bulbs that had the glass covers removed.7 These sparked and ignited the IEDs’ contained explosives.8 According to one report, the devices may have cost as little as $100 to build.9

Improvised explosive devices are easy to make, difficult to combat, and cause significant harm and disruption. Militant IED attacks have caused countless civilian and troop casualties in conflict zones abroad. Likewise, they are responsible for considerable death, destruction, and panic within the United States. Major ongoing conflicts, such as the war in Ukraine and the recent surge of hostilities in Israel and Gaza, can serve as highly publicized testbeds for novel means of IED development and employment. Because of their outsized impact—as well as the wide availability of device components—violent extremists continue to consider IEDs a valuable part of their arsenal.

Yet, the expected persistence of a threat should not be mistaken for rote repetition. The terrorist IED threat is not static. Violent extremists continue to innovate, drawing on emerging technologies and creative problem solving. In response, law enforcement, military, and intelligence practitioners devise new preventative and interdiction methods. Terrorists respond by finding novel ways to conduct successful attacks. The cycle repeats itself. As a result, despite billions of dollars invested in the counter-IED mission over the past 20 years, yesterday’s solutions are at risk of being poorly suited to tomorrow’s threats.

The onus of initiative requires that practitioners and applied researchers look over the horizon to identify emerging threats. In support of this need, the authors offer a forward-leaning taxonomy of emerging threats related to terrorist use of IEDs in the United States and consider its implications. The article proceeds as follows. First, the authors provide a framework with which to conceptualize and classify IEDs and IED-related incidents. Next, they briefly analyze recent global trends in terrorist use of explosives. Third, they identify key emerging threats related to the threat actors, methods, and targets connected to IED-related incidents in the United States. Finally, the article concludes with a discussion on the implications that these emerging threats may hold for the counter-IED mission community, supporting research efforts, and U.S. national and homeland security.

A Rapid-Fire IED Taxonomy
Although IEDs are not new, the term “improvised explosive device” only emerged as recently as the 1970s. It has since been tethered to a wide array of definitions and taxonomies. These occasionally differ regarding the nature of device components, the source of a device’s “improvised” nature, and the intended use by whomever is conducting an attack. The lack of consistency is driven partially by the fact that practitioners and academics both heavily contributed to this debate with limited communication across disciplines and with distinct purposes behind them.

Most academic definitions contain a broader selection of inclusionary characteristics, encapsulating the development process, ingredient types, desired effect from usage, and types of adversaries that may or may not use this method of attack. Prioritizing inclusivity and practicality as guiding principles, Paul Gill and his colleagues synthesize nearly 30 different definitions to offer the following conceptualization:

An explosive device is considered an IED when any or all of the following — explosive ingredient, initiation, triggering or detonation mechanism, delivery system — is modified in any respect from its original expressed or intended function. An IED’s components may incorporate any or all of military grade munitions, commercial explosives or homemade explosives. The components and device design may vary in sophistication from simple to complex and IEDs can be used by a variety of both state and non-state actors. Non-state actors can include (but not be limited to) terrorists, insurgents, drug traffickers, criminals and nuisance pranksters.10

Practitioners face a similar issue. IED taxonomies can vary considerably across different elements of the counter-IED mission community (e.g., military Explosive Ordnance Disposal technicians, intelligence community analysts, local bomb squad units, and federal law enforcement officers). The lack of a standardized and universally adopted framework—the systematic description of key device components—can inhibit the data collection, information sharing, and synchronized implementation needed to mitigate current and future IED-related threats.

While IEDs are inherently bespoke, most modern devices feature five basic component types: (1) a switch, (2) an initiator, (3) a main charge, (4) a power source, and (5) a container. Figure 1, taken from the United Nations Mine Action Service, summarizes this IED technical categorization framework.11

Some IEDs also make use of enhancements, which terrorists add to a device to increase its physical or psychological effects. Common enhancements include shrapnel (e.g., nails, screws, or ball bearings) and fuel (e.g., propane or other gas). Terrorists could create “dirty bombs” by adding chemical, biological, radiological, or nuclear (CBRN) agents.12 In 2014, the Islamic State successfully enhanced an unknown number of IEDs delivered via aerial and land vehicles, as well as roadside victim operated IEDs, with chlorine and mustard gas.13 Reportedly, the Islamic State had access to university laboratories in Mosul and chemical experts to conduct these attacks.14

A clear, concise conceptualization and taxonomy of IEDs are necessary to identify and characterize emerging threats in this space. Looking ahead, one can expect to see terrorists innovate around IEDs in two general ways: first, in the component composition of the device itself, and second, in the methods by which they plan and conduct IED-based attacks. In the next section, before elaborating on emerging trends in the United States along these dimensions, the authors summarize recent trends in IED-related terrorist activity.

Recent Global and Domestic Trends in Terrorists’ Use of IEDs
The IED threat is, regrettably, alive and well. A recent report by Action on Armed Violence (AOAV) identified a total of 640 incidents worldwide involving IEDs across 33 countries and territories from January to June 2023.a These attacks were responsible for a recorded 1,456 civilian casualties, including 450 deaths. Notably, after years of steady decline in IED-based violence, the rates reported for this year thus far portend a possible increase in IED use for the first time since 2018.

It is unsurprising that improvised explosives continue to be one of the most common tools and methods used by terrorists and militant organizations abroad. As tactical instruments, IEDs are capable of wielding outsized, even strategic, effects. The widespread use of IEDs by local insurgent forces in Afghanistan and Iraq against U.S. forces, for example, often compelled deployed troops to restrict themselves to armored vehicles, avoid key travel routes, and advance at a snail’s pace through sensitive areas.15 And with justified cause. By some estimates, at least half of American troop fatalities in Iraq as well as in Afghanistan were caused by IEDs.16 b IEDs can quickly level an asymmetric playing field.

Groups and individuals associated with the Islamic State and al-Qaida continue to drive much of IED-related activity around the world, accelerated by both organizations’ ongoing regional expansion. Notably, al-Qaida affiliate al-Shabaab has long relied on IEDs as a critical tool against both civilian and military targets in Somalia.17 Now an operational trademark of the group, combatants often integrate vehicle borne IEDs (VBIEDs) and/or person borne IEDs (PBIEDs) in multi-stage, complex ground assaults.18 Al-Shabaab operatives were reportedly responsible for the recent surge of IED-based attacks around key border areas in Kenya, such as Garissa and Lamu counties.19 Observers have noted a similar, concerning trend by violent extremist forces in the Sahel and West Africa.20

Other global regions are also experiencing sustained or increased IED activity. Militant actors in Afghanistan and Pakistan continue to perpetrate IED attacks at a high tempo. In a June 2023 report, the UN Assistance Mission in Afghanistan’s Human Rights Service reported that 3,774 civilians were seriously injured or killed by conflict actors between August 2021 and May 2023.21 Roughly 75 percent of those recorded casualties are attributed to militant use of IEDs in crowded public spaces, including houses of worship, schools, and markets.22 In the immediate aftermath of the Taliban takeover in August 2021, IED attacks by the Islamic State’s affiliate in Afghanistan, Islamic State Khorasan Province (ISK), increased significantly, including an uptick in observed suicide PBIEDs attacks.23 Drawing on an original dataset of extremists charged in U.S. courts for their roles in planning or perpetrating attack plots tied to the Afghanistan-Pakistan region, Andrew Mines found that IEDs have been the most frequent method of attack between 1985 and 2023.24

The IED threat is hardly limited to foreign wars or political unrest abroad. This issue hits home for many Americans. Of course, there are the devastating and well-documented domestic terrorism incidents, such as the 2013 Boston bombing and the 1995 Oklahoma City bombing. Looking back further into American history, explosives-based attacks emerge as a regular feature of domestic extremist activity: Consider the Weather Underground bombings in the 1970s, the 16th Street Baptist Church bombing of 1963, and the 1920 anarchist bombing of the J.P. Morgan Building on Wall Street, for example.25 Audrey Cronin records a staggering 216 unique bombing incidents between 1867 and 1934 in the United States as political dissidents of various kinds made regular use of newly developed and widely accessible dynamite.26 In short, ideologically motivated actors in America have long used explosives to coerce outgroups or challenge the political status quo. And the threat remains. For example, one day before the January 6 insurrection, a still unidentified individual left two pipe bombs outside the Republican National Committee and Democratic National Committee headquarters on Capitol Hill. The devices did not detonate.27 Even more recently, amid concern that there could be a resurgence in jihadi terrorism in the West because of the war in Gaza, in October 2023 a Jordanian man living in Texas was arrested after allegedly posting online about his support for killing Jews and viewing “specific and detailed content posted by radical organizations on the internet including lessons on how to construct bombs or explosive devices.”28

While there have been relatively few successful terrorist bombings overall, U.S. intelligence and law enforcement efforts have successfully thwarted several local IEDs plots and attacks. In August 2023, for example, authorities arrested a 17-year-old resident of Philadelphia, Pennsylvania, alleging “he was preparing to build bombs and select targets after being in touch with an al-Qaida affiliate in Syria.”29 The suspect’s phone messages and internet history revealed instructions for how to make improvised explosive devices. Additional surveillance found that the teen had purchased materials to make the bombs in the weeks prior to his arrest, including chemicals, wiring, and devices that could be used as detonators.30 In addition, U.S. Customs and Border Protection provided records revealing 14 international shipments of military tactical gear to the teen’s home address.31 The case is ongoing.

In January 2011, Kevin Harpham, a white supremacist affiliated with the National Alliance, admitted to placing an IED along the route of a Martin Luther King parade in Spokane, Washington.32 The device, a shaped charge, was designed to scatter shrapnel covered with rat poison to keep victims’ wounds from coagulating.33 The IED was discovered less than an hour before the parade started and was subsequently disarmed.34 In February 2020, Demetrius Nathaniel Pitts, a U.S. citizen who had pledged his allegiance to al-Qa`ida, was sentenced for plotting to bomb a July 4th parade in downtown Cleveland in 2018.35 Pitts planned to use both remote-control cars filled with explosives and loaded with metal shrapnel and a larger vehicle packed with explosives to “cause maximum damage.”36 The attack was stopped by an FBI employee acting in an undercover capacity.37

In a more recent example, in February 2022, Christopher Cook, Jonathan Frost, and Jackson Sawall pleaded guilty to conspiring to conduct a domestic terror attack—one that presumably, if not foiled, would have involved the use of explosives.38 The men had met on Iron March, a neo-Nazi online forum, and joined forces. Federal prosecutors argued that the group’s intended target for a large-scale attack was the U.S. power grid “to stoke division in furtherance of white supremacist ideology.”39 As part of the conspiracy, each member of the group was reportedly assigned a substation in a different region of the United States.40 Their group used an encrypted messaging app to share information with one another about gathering firearms and explosives for use in the attack. And, in an August 2020 search of the defendants’ homes, the FBI found precursor chemicals “which could be used to create an explosive device.”41 Cook and Frost were sentenced in April 2023.42

It is worth mentioning that the future IED threat in the United States may also stem from conspiracy-based ideologies, some of which may have links to violent extremist beliefs but do not necessarily on their own constitute domestic terrorism. On Christmas Day 2020, a vehicle borne IED exploded in downtown Nashville, Tennessee. The bombing occurred outside of an unmarked communication building and, according to one report, caused extensive communications and power outages, flooding, and a fire within the building.43 Federal law enforcement determined that the action was not related to terrorism, but the investigation team notes that the perpetrator, Anthony Quinn Warner, maintained “long-held individualized beliefs adopted from several eccentric conspiracy theories.”44

In summary, despite the relatively slow cadence of successful domestic bombings of late, the authors warn against laxity—the IED threat is not going anywhere. A declassified U.S. military report on improvised methods utilized by Viet Cong forces describes the complexity of observed IED-based attacks as being “only limited by the ingenuity of the man who constructs them.”45 More than 50 years later, this is no less true. The near-endless combinations of IED design (i.e., technical categorization) and application (i.e., tactical characterization) continue to present a significant challenge to the counter-IED mission community.

When terrorists and violent extremists innovate, they gain an advantage, even if temporarily, in circumventing existing detection and interdiction methods. Therefore, anticipating the timeline and locus of innovation is essential to maintain a proactive posture in countering terrorist use of explosives. In the next section, the authors describe how the terrorist IED threat is evolving and outline a five-part framework for classifying related emerging threats.

Emerging Threats in Terrorist Use of IEDs in the United States
The authors anticipate a number of emerging threats related to the methods and targets of IED-based plots and attacks in the United States.46 Anticipating how terrorists might attack in the future—and why—is central to the design of efforts to protect the security of both individual targets and the nation as a whole. Some of the emerging threats discussed here represent incremental evolutions, indicating areas for targeted adjustment within the existing counter-IED infrastructure.47 Others point to potential structural shifts in the terrorism milieu, requiring deeper thought and potential responsive action around how the interagency counter-IED mission community organizes its future efforts.

The authors identify five overlapping categories that they expect to act as underlying currents propelling the future explosives threat: the threat actor landscape, technical categorization, tactical characterization, malign use of emerging technologies, and the target surface. Each emerging threats category features key indicators or propellants, which will be discussed. Combined, these categories speak to critical questions: Who is likely to use IEDs, how are they likely to develop and employ IEDs, and where will IED attacks be targeted? Importantly, this set of related analytic categories transcends the boundaries exclusive to a single critical mission area—such as prevention, detection, or render safe—and creates an opportunity for cross-cutting insights.

  1. The Threat Actor Landscape in the United States
    The terrorism milieu in the United States is increasingly fragmented. It is a problem of addition, not substitution. Jihadi terrorism still presents a significant threat to the United States and its partners, especially through homegrown violent extremism (HVE). The simultaneous rising activity of various domestic violent extremists—such as racially and ethnically motivated violent extremists (RMVE) and anti-government/anti-authority violent extremists (AGAAVE)—in the United States has led to a structural shift in the threat actor space. It is ideologically diverse and geographically diffuse. While some foreign terrorist organizations likely maintain a capability and intent of striking the U.S. homeland, the greatest likelihood of an attack in the United States comes from within.c

Domestic violent extremists and homegrown violent extremists showcase significant range in their ideological agendas, espoused grievances, and general relationship to the status quo. But as a tactical tool, the IED is more ecumenical and will have a catholic appeal across tomorrow’s threat actor ecosystem. Instructions for the construction of IEDs and the call for their use are featured strongly in salafi-jihadi publications, such as Inspire; white supremacist RMVE channels, such as Terrorgram; and anarchist violent extremists’ canon texts, such as The Anarchist Cookbook.48 As such, the general use of IEDs or specific TTPs reflected in observed plots and attacks may not necessarily vary meaningfully across this fragmented actor landscape.

However, the fragmentation of the threat actor landscape in the United States—compounded by the simultaneous presence of organized violent extremist groups, decentralized cells, and seemingly “lone actors”—may shape where and why one expects to see explosives used in domestic terrorist attacks.49 Some may focus on accelerating “mayhem,” i.e., overthrowing or destabilizing the prevailing political and social order.50 Others clearly remain intent on generating mass casualties. For extremists intent on using explosives, their respective ideologies may shape their target selection.51 For example, drawing on federal court documents from 2016 to 2022, one report finds evidence that salafi-jihadi and white supremacist attack planners have largely targeted different U.S. critical infrastructure sectors, with “the former focusing on the commercial facilities, government facilities, and emergency services sectors, and the latter predominantly focusing on the energy sector.”52 This suggests distinct underlying logics—corresponding with different ideological foundations and related objectives—behind the use of IEDs by violent extremists in the United States.

As a result, rather than one target profile in IED attacks, there will likely be multiple. Different types of targets (e.g., public crowded spaces, the power grid, etc.) will likely carry unique symbolic or practical value for different sorts of U.S.-based violent extremists.53 Of particular concern is the rising number of threats, plots, and attacks against U.S. public officials and elected representatives. This trend has been mostly driven by anti-government anti-authority violent extremists, though not exclusively.54 Many such incidents in the future may involve IEDs, an instrument firmly rooted in the history of political assassinations. In August 2023, for example, James Clark pleaded guilty to sending a bomb threat to an election official in the Arizona Secretary of State’s office. Clarke warned the official that she needed to “resign by Tuesday February 16th by 9 am or the explosive device impacted in her personal space will be detonated.”55 According to federal investigators, the would-be attacker had conducted online searches that included the full name of the election official, instruction on how to kill, the official’s residential address, as well as details on the Boston Marathon bombing.56 This presents a fundamentally different challenge than protecting the homeland from another 9/11 type attack. It points to the risk of normalization of political violence and an erosion of civic norms – all propelled by the increasingly diverse set of violent extremists present in the United States.57

  1. IED Technical Categorization
    Terrorists have ample opportunity to innovative in the design and manufacture of IEDs through its various components. A focus on patterns of IED technical categorization (i.e., a systematic construct of an IED’s components; see Figure 1) can provide useful leverage over emerging trends in terrorist use of explosives. The communication and diffusion of novel inventions, which may originate from terrorist bombmaking camps, basement labs, or benign hobbyist communities, reduce barriers of entry for would-be attackers and increase challenges for interdiction.

When developing an IED, most terrorists in the United States have relied on commercial explosives such as propellants (e.g., black and smokeless powders) and pyrotechnics (e.g., flash powders) as main charges or use readily available precursors chemicals such as peroxides and fuel-oxidizer mixtures (e.g., ammonium nitrate-fuel oil) to manufacture homemade explosives (HMEs).58 The use of military explosives (e.g., Semtex) is exceptionally rare in U.S. bombing incidents. In general, the set of common IED main charges has been relatively unchanged over the past 15 years and is unlikely to shift significantly in the coming years.d However, current limitations in detecting inorganic species and complex mixtures, such as homemade fuel-oxidizer explosives, across a range of environmental conditions, continues to present challenges and represents part of the still evolving threat.

Another emerging threat stemming from IED technical categorization is presented by a different key device component: the switch. Significant advances in electronics over the past few decades have enabled terrorists to make creative changes to device switches. These advancements matter; how a device is commanded or activated is a critical part of an attack plan that determines where the responsible terrorist(s) may be at the time of the bombing. It affects the set of viable prevention, detection, and defeat options available to the counter-IED mission community. Electronic triggers have evolved from the use of Casio watches to radio transmitters and from garage door openers to mobile phones. Timing chips sell for a few dollars, and small computing chips sell for well under $100, making such materials inexpensive to acquire. These advancements open the field for command, time, and victim-operated switches, standing to increase the operational range and fidelity of terrorist IED attacks. In particular, the authors observe the emerging potential opportunity for devices controlled or activated via WiFi as well as the potential for increased use of sensor-based switches to facilitate more sophisticated or targeted attacks.

Lastly, it is worth noting that terrorists constantly imagine and adopt new containers both for the confinement of the main charge but also for the purpose of concealment. Metal pipes and pressure cookers are among the most common containers used in U.S. attacks, but creativity abounds. Recent incidents, for example, have seen terrorists in the United States or abroad use handheld radios, printers, synthetic rocks, aluminum drink cans, and other non-descript items as containers.59 The sheer scope of possible containers creates Herculean challenges to forecasting—and therefore mitigating—potential trends in their adoption.

  1. IED Tactical Characterization
    Terrorists continue to show great creativity in the tactical characterization of IED-based attacks (i.e., the way in which an IED incident is planned and conducted). This can be assessed across multiple phases of the terrorist attack sequence, including operational preparation and device employment.

A major contributor to—if not a key enabler of—the evolving IED threat is the sharing and dissemination of malign tactics, techniques, and procedures (TTPs) through online platforms and communication channels, such as YouTube, Facebook, WhatsApp, and microblogs.60 For would-be attackers with minimal prior experience or knowledge, the dissemination of best practices and novel solutions lowers barriers of entry to building a functional IED. It is not only the wide dissemination of articles like “How to Build a Bomb in the Kitchen of Your Mom.”61 It is also dissemination of TTPs at scale through online micro-engagements enabled by increased user technological savvy and an ever-widening range of platform options. This risk was specifically reinforced in the 2023 Europol Terrorism Situation and Trend Report, which assessed that “terrorists and violent extremists remain apt in evading restrictive measures and monitoring related to explosive precursors in the EU. For instance, a pro-IS group released a document on a cloud-based instant messaging platform, suggesting the use of alternative precursors for HMEs.”62 Further compounding this threat, the advents of the metaverse and Web3 offer further opportunities through obscured website launch, file‑hosting services, and discrete communication for evasion of present content moderation and monitoring efforts.63 This, paired with the exploitation of readily available off-the-shelf technologies, means that violent extremists are able to both develop and employ IEDs with a relatively small investment and higher degree of reliability.64

Employment—the means by which IEDs are delivered to a target—is a major component of an IED attack tactical design. One of the highest tempo evolutions in the IED threat area is found in the potential for delivery of explosives via unmanned systems (UxS).65 This threat is still evolving.66 Unmanned vehicles can range from a toy-radio-controlled boat or car to a quadcopter aerial drone to an autonomous ground- or surface- based vehicle. Thus far, the greatest attention has been given to unmanned aerial systems. In November 2012, Rezwan Ferdaus was sentenced for plotting an attack in support of al-Qa`ida on the Pentagon and the U.S. Capitol using a model aircraft filled with C-4 plastic explosives.67 As reported by FBI investigators involved in the case, the would-be attacker was the mastermind behind the operational plan: “Mr. Ferdaus’ sentence reflects that he alone conceived the plot, was responsible for his illegal acts, and acted purposefully.”68 To be sure, this plot was dependent on the supply of explosive materials by an FBI undercover employee, suggestive of some control over this threat incident.69 However, the highly publicized spike in the use of armed unmanned aerial vehicles in Syria and Iraq by the Islamic State, and possibly the ongoing central role of small UAS in the Ukrainian resistance, may inspire a greater frequency of similar attempts by terrorists in the United States.70 In February 2022, FBI Director Christopher Wray reported to the U.S. Senate Homeland Security and Governmental Affairs Committee that the FBI was “investigating, even as we speak, several instances within the U.S. of attempts to weaponize drones with homemade IEDs. That is the future that is here now.”71

In January 2023, a graduate student in the United Kingdom was arrested for building a drone to deliver a bomb on behalf of the Islamic State.72 The student had filled out an Islamic State application form. During the arrest, authorities seized several devices. A 3D printer was also found at the property.73 The case is ongoing, but it indicates that the tactics developed and honed in conflict zones abroad may be adopted by like-minded persons living in otherwise peaceful environments. Compounded by the concurrent advancement of other emerging technologies, terrorist use of unmanned systems is an evolving threat that will continue to present a host of novel challenges to current security infrastructure and protocols.74

  1. Malign Use of Emerging Technologies
    Serving as a force multiplying function to a number of the threats identified above, the malign use of emerging technologies provides feasible means of novel IED design and employment. Of notable concern is that such emerging technologies are rarely military systems, but rather commercial off-the-shelf. As these become more affordable, more reliable, and widely available, the threat they present will only become greater.75

Although hardly exhaustive, the authors offer a handful of emerging technologies that carry notable implications for the future IED threat. The growing knowledge and use of microcontrollers (e.g., Arduino Uno) and single-board computers (e.g., Raspberry Pi) present a worrying new means of device construction, with disquieting implications for their application.76 They provide a low-cost, easy way for novices and experts alike to construct IEDs that interact with motors and sensors that may measure light, temperature, pressure, and other environmental variables.77 Adding to this threat, the lawful online activities carried out by communities of hobbyists and in “maker spaces” around these technologies is a growing avenue through which individuals and groups with malign intent may become sufficiently knowledgeable and skilled in their use with relatively little cost or time.

Relatedly, the sustained advancements in infrastructure surrounding the Internet of Things (IoT), including high-speed internet and 5G/6G telecommunication networks, open a new threat vector related to IED-based attacks.e Extending the logic of a radio-controlled device, a WiFi-connected IED could be controlled through an extensive array of methods, and from a much wider geographic range. As a recent report on IED threats put it: “We are at a stage … where the ‘internet of things’ offers an endless choice of switches.”78 If addition, a device’s integration into the IoT may also support the employment of an IED device through the operation of an autonomous vehicle or unmanned system (UxS).

In another vein, the past year has seen a surge of public use of generative artificial intelligence (AI) through the release of ChatGPT, DALL-E, and other programs built around machine learning-based models. Opportunities for malign use abound. In the realm of IEDs, generative AI may provide new avenues for the development of explosives, the identification of attack targets, operational planning, training, and other key tasks. To be sure, it is unlikely that generative AI will wash out a dependency on individual expertise, but it will help to level the playing field, reducing the time required for any given individual to develop viable homemade explosives and improvised devices.

Finally, rapid and significant advancements in additive manufacturing (AM) (i.e., the process of creating a three-dimensional object by building up a series of successive layers of material(s) over time) are a key part of the evolving IED threat landscape. Recent efforts, for example, have demonstrated the possibility of using 3-D printing to create elements of energetic materials applicable to high explosives, propellants, and pyrotechnics.79 These could presumably be used in device boosters or main charges and, in some cases, help aspiring attackers to circumvent current prevention barriers. Logistically, AM may reduce time sensitivities, provide opportunities for rapid prototyping, and circumvent supply-chain constraints—all in an operationally secure environment. These functions reduce common barriers to developing IEDs. Additive manufacturing also opens a new avenue for crafting IED containers and other key device components in a way that is both low cost and discrete.

  1. Expanding Target Surface
    The target surface for IED attacks is complex and varied, but the authors observe several trends regarding the targets that may appeal to threat actors in tomorrow’s political environment. This is important as threat assessments, intervention efforts, and preventative security measures will likely differ for different target types, albeit with some overlap. Among these include U.S. critical infrastructure, crowded spaces, and public officials.

Critical infrastructure sites such as power grid substations, transportation systems, and crowded spaces are among the most frequently targeted soft targets for threat actors. Earlier this year, the U.S. Department of Homeland Security identified rising threats to critical infrastructure as a leading national risk. For example, violent extremists are likely to maintain the growing operational focus on the U.S. power grid; neo-Nazi extremists Brandon Russell and Sarah Beth Clendaniel conspired to conduct a series of “sniper attacks” on Maryland electrical substations earlier this year.80 In addition to transportation and energy, other national critical infrastructure sectors are also generally seen as attractive targets for extremist attacks, including communications, food and agriculture, and healthcare and public health. These incidents tend to garner mass attention and cause significant harm and disruption, aligning with the doctrine of multiple elements of today’s violent extremist ecosystem. Extremists may use a wide range of TTPs for such attacks, including the use of IEDs.

As expressed in a recent government threat advisory bulletin, the terrorism threat may originate from a wide range of ideologically motivated individuals or groups and could focus on a multitude of soft targets: mass transit areas, arenas, shopping malls, open-area gatherings, houses of worship, and others.81 Crowded public spaces, including large public venues, present one of the greatest challenges for detection and protection owing to their numerous entrance and egress points, open spaces, and lack of security by design. For terrorists looking to maximize casualties, unrestricted public areas are ‘low-hanging fruit.’

Public officials are also an increasingly frequent target of violent extremist activity. A long-standing tool of assassination, explosives were used in the earliest phases of modern terrorism by violent extremist groups such as Narodnaya Volya.82 A growing number of plots are targeting U.S. elected officials and federal law enforcement.83 Notably, these threats may come from across the violent ideological spectrum.”84 In May 2023, Jessica Higginbotham, a former security subcontractor working at the Athens-Clarke County Democratic Party campaign office, pleaded guilty to threatening to bomb their local headquarters while the U.S. Senators from Georgia were in town for campaign events. In August 2018, Venezuelan President Nicolás Maduro survived an assassination attempt involving explosive drones.85 Based on the open-source information available, it appears that an attack involved two drone-borne IEDs while Maduro was giving a speech in Caracas, celebrating the 81st anniversary of the National Guard.86 Given widespread access to commercial-off-the-shelf systems and the rising rate of threats and plots against U.S. public officials, the authors anticipate that similar attempts may be made by ideologically-motivated actors in the United States in the future.

Looking Ahead
IEDs are easy to make, difficult to combat, and cause disproportionate harm and disruption. In 2015, terrorism and insurgency expert David Kilcullen remarked “I sometimes hear people express the hope that the IED threat will diminish as Western forces pull out of Afghanistan. Unfortunately, nothing could be further from the truth—the IED has now entered the standard repertoire of irregular forces in urban areas across the planet, and there are no signs this threat is shrinking; on the contrary, it seems to be growing.”87

Over the past decade, there has been a relative increase in violent extremist activity across the United States. Yet, successful bombings have been relatively few. Given the steady cadence of attempted IED attacks, however, motive or intent does not seem to be waning. The authors, therefore, warn against a false sense of security. The fact that intelligence and law enforcement practitioners have thwarted so many of the recent attack plots in the United States may indicate a future risk that terrorists will be oriented toward developing novel means of IED development and employment.f This would pose new challenges to those responsible for combating terrorist use of explosives.

The U.S. domestic counter-IED mission community is currently organized around five critical mission areas: deterrence, prevention, detection, protection, and response.88 These are suitable categories for action, but less useful as analytic concepts. Far too many emerging threats bleed across the missions’ boundaries. Thus, to support proactive assessment and mitigation, the authors have presented a five-part framework for identifying and assessing emerging threats related to the development of IEDs and their employment. They argue that there is value in dissecting key features of this evolving threat, including attention to the threat actor landscape, IED technical categorization, IED tactical characterization, malign use of emerging technologies, and the target surface.

Looking ahead, and thinking about next steps, the authors offer three guiding principles for practitioners and academic researchers as they work to proactively identify actionable, sustainable, and durable solutions to the evolving IED threat.

  1. Inhibit terrorist access to IED materials. This means reducing malign actors’ acquisition and use of precursor chemicals and known bomb-making materials. This work is already well underway as a critical mission area,g but the authors’ analysis suggests the need for additional considerations. Based on the set of emerging technologies identified here, for example, law enforcement and first responders should include small UxS, microcontrollers and single board computers, 3-D printing systems, and such components on lists of potential plot-warning indicators. A focus on this goal will also require monitoring the avenues by which terrorists disseminate TTPs, best practices, and general knowledge on the development of HMEs and the construction of IEDs. As discussed, much of this is occurring online. Microblogs and Web3, social media platforms, encrypted message applications, and the emerging metaverse will all be viable points of access for lethal ‘know how.’
  2. Disrupt IED facilitation networks and IED plots. In some cases, efforts to restrict terrorist access to precursor and bomb-making materials will be unsuccessful. The next objective should be intercepting potential attackers and disrupting their facilitation networks. This demands a clear understanding of targets at risk, probable locations for sourcing and IED development, tactics, techniques, and procedures (TTPs), and the actors likely to be behind an attack. In addition to maintaining robust intelligence and law enforcement capabilities, this requires deep knowledge on both established and emerging trends. Given the changing nature of the threat actor landscape and an expanding target surface, the authors see notable value in an updated analysis of the individual- and environment-level precedents of IED adoption and tactical design. More generally, analytic initiatives should be oriented on future, anticipated challenges. Future-focused methodologies such as horizon scanning, predictive scenario-simulation analysis, and/or risk assessment evaluations may be especially useful toward bridging the gap between previous and anticipated manifestations of the IED threat and developing forward-leaning solutions focused on future plots.
  3. Enhance interagency and inter-sector coordination. From 2006 to at least 2015, a large part of the counter-IED effort—and connected federal research and development funding—was driven by military needs based on U.S. activity in Afghanistan, Iraq, and other theaters. The Joint IED Defeat Organization (JIEDDO) was a gravitational center of this effort, boasting more than $18 billion in funding over much of that period.h But these are leaner times. As defense priorities shift from counterterrorism to great power competition and as violent extremist activity increases domestically, “much of the U.S. prevention and counterterrorism workforce will be expected to do more with less as they hold the line.”89 Under these circumstances, effective action to counter emerging and evolving IED threats will require clearly identified roles and responsibilities and formal mechanisms of information sharing, coordination, and collaboration across the interagency mission community. Moreover, robust partnerships between government practitioners, industry leaders, and academic experts will be constructive toward developing and implementing actionable and evidence-based mitigating strategies.

While this article largely focuses on the potential implications of this evolving threat for U.S. national and homeland security, much of the discussion presented here will also pertain to other countries including across Western Europe, Canada, Australia, and New Zealand as well as certain hostile environments in the Middle East, Central and South Asia, and sub-Saharan Africa.

The desired end state is the safeguarding of local communities, the protection of critical infrastructure, and the minimization of harm in cases of successful IED attacks. Combating future terrorist use of IEDs in the United States and elsewhere necessitates a frank assessment of where the IED threat is heading, where existing security measures are sufficient, and where the current counter-IED architecture may require a remodel to match the changing times.

Substantive Notes
[a] Regarding the AOAV report methodology: Because attribution is not always possible, it is possible that some of the recorded incidents may not reflect activity by armed non-state actors. However, militants are recorded as the most common perpetrators of IED attacks, specifically the Islamic State, al-Shabaab, and the Baloch Liberation Army. See Chiara Torelli, “Explosive Violence Monitor 2022,” Action on Armed Violence, 2023; Iain Overton, “Report on IED Incidents for January-June 2023,” Action on Armed Violence, July 13, 2023.

[b] An estimated 16,500 IEDs were “detonated or discovered being used against U.S. forces in Afghanistan in 2011” alone. “Counter-Improvised Explosive Devices: Multiple DOD Organizations are Developing Numerous Initiatives,” GAO-12-861R, U.S. Government Accountability Office, August 1, 2012.

[c] According to a recent U.S. Government Accountability Office (GAO) report on domestic terrorism, over the past decade, RMVE actors have been responsible for a 357 percent increase in domestic terrorism-related cases, accounting for many of the most violent incidents from 2010 to 2021. See “The Rising Threat of Domestic Terrorism in the U.S. and Federal Efforts to Combat It,” U.S. GAO, March 2, 2023, as well as Don Rassler, The Compound Era of U.S. Counterterrorism (West Point, NY: Combating Terrorism Center; Tampa: Joint Special Operations University, August 2023).

[d] In 1998, the National Academy of Sciences (NAS) National Research Council issued its Containing the Threat from Illegal Bombings report. In 2018, NAS published Reducing the Threat of IED Attacks by Restricting Access to Chemical Explosive Precursors. Comparison of these reports and additional data published by the U.S. Bomb Data Center confirms that the set of precursor materials assessed most likely to present a threat remained relatively unchanged over that 20-year period. Explosive materials follow a similar trend: The use of pyrotechnics and fireworks continue to lead explosive instances in the United States by a sizable margin.

[e] As defined by the U.S. Cybersecurity and Infrastructure Security Agency, the Internet of Things (IoT) refers to “any object or device that sends and receives data automatically through the Internet. This rapidly expanding set of ‘things’ includes tags (also known as labels or chips that automatically track objects), sensors, and devices that interact with people and share information machine to machine.” See “Securing the Internet of Things (IoT),” Cybersecurity and Infrastructure Security Agency, February 1, 2021. See also Zakria Qadir, Khoa N. Le, Nasir Saeed, and Hafiz Suliman Munawar, “Towards 6G Internet of Things: Recent advances, use cases, and open challenges,” ICT Express 9:3 (2023).

[f] For example, some reports suggest that the Islamic State may have responded to new counter-drone measures by designing their systems “in such a way as to reduce their radar signature” including wrapping their drones in tape to avoid detection. See Arthur Holland Michel, “Counter Drone Systems,” Center for the Study of the Drone, December 2019; Austin C. Doctor, “The Militant Drone Threat is No Longer New. Why Does It Still Feel Novel?” Modern War Institute, West Point, February 2, 2022.

[g] For example, the U.S. Cybersecurity and Infrastructure Security Agency Office for Bombing Prevention manages the Bomb-Making Materials Awareness Program. See “Counter-IED Resource Guide,” CISA Office for Bombing Prevention, March 2017. For an example of a similar program in the European Union, see “Regulation EU 98/2013 on the marketing and use of explosives precursors,” European Union Migration and Home Affairs, May 24, 2017. The U.K. Home Office requires that “Members of the public who want to import, acquire, possess or use these chemicals must hold an Explosives Precursors and Poisons (EPP) licence issued by the Home Office and an associated photographic identity document.” See “Guidance: Supplying explosives precursors and poisons,” United Kingdom Home Office, August 15, 2023.

[h] The JIEDDO mission has been re-organized but continues. “In March 2015, after considering a range of options, the Deputy Secretary of Defense designated JIEDDO a combat support agency focused on counter-terrorism, counter- insurgency, and other related operational areas, including counter-IED. In April 2015, JIEDDO’s name was changed to JIDA to reflect this expanded mission. In February 2016 DOD renamed JIDA to JIDO and placed it under the authority of the Director, DTRA, effective October 1, 2016.” For more, see “Warfighter Support: DOD Needs Strategic Outcome-Related Goals and Visibility over Its Counter-IED Efforts,” U.S. GAO, GAO-12-280, February 22, 2012.

Citations
[1] Jonathan D. Gates, Sandra Arabian, Paul Biddinger et al., “The Initial Response to the Boston Marathon Bombing,” Annals of Surgery 260:6 (2014): pp. 960-966; Michael Casey, “Boston Remembers Deadly Marathon Bombing 10 Years Later,” Associated Press, April 15, 2023.

[2] “After Action Report for the Response to the 2013 Boston Marathon Bombings,” Massachusetts Emergency Management Agency, December 2014.

[3] “Unclassified Summary of Information Handling and Sharing Prior to the April 15, 2013 Boston Marathon Bombings,” Office of the Inspector General, April 10, 2014; Judy Woodruff, “Bombing Suspect Alleges Attack Was Self-Motivated, Not Connected to Other Groups,” PBS NewsHour, April 23, 2013.

[4] Azmat Khan, “The Magazine That ‘Inspired’ the Boston Bombers,” Frontline, April 30, 2013.

[5] United States v. Tsarnaev, 1:13-cr-10200, (D. Mass. Jun 27, 2013) ECF No. 58; “Human Behavioral Responses After a Targeted IED Attack at Soft Targets and Crowded Spaces,” U.S. Department of Homeland Security, forthcoming.

[6] “Reducing the Threat of Improvised Explosive Device Attacks by Restricting Access to Explosive Precursor Chemicals: Key Recommendations,” National Academies of Sciences, Engineering, and Medicine, 2018.

[7] Jane Hunter, “Blood on the Streets of Boston: Reviewing the Response to the April 2013 Marathon Bombings,” Action on Armed Violence, December 2014.

[8] Becky Bratu and M. Alex Johnson, “Tsarnaevs Used Christmas Lights in Boston Marathon Bombs: Documents,” NBC News, May 22, 2014.

[9] Joby Warrick and Sari Horwitz, “Boston Marathon Bombs Had Simple but Harmful Designs, Early Clues Indicate,” Washington Post, April 16, 2013.

[10] Paul Gill, John Horgan, and Jeffrey Lovelace, “Improvised Explosive Device: The Problem of Definition,” Studies in Conflict & Terrorism 34:9 (2011): pp. 732-748.

[11] “Improvised Explosive Device Lexicon,” United Nations Mine Action Service, July 2016.

[12] Piotr W. Sielicki, Mark G. Stewart, Tomasz Gajewski et al., “Field test and probabilistic analysis of irregular steel debris casualty risks from a person-borne improvised explosive device,” Defence Technology 17:6 (2021): pp. 1,852-1,863.

[13] Columb Strack, “The Evolution of the Islamic State’s Chemical Weapons Efforts,” CTC Sentinel 10:9 (2017).

[14] Margaret Coker and Ben Kesling, “Islamic State Hijacks Mosul University Chemistry Lab for Making Bombs,” Wall Street Journal, April 1, 2016.

[15] Jason Shell, “How the IED Won: Dispelling the Myth of Tactical Success and Innovation,” War on the Rocks, May 1, 2017; “S. Hrg. 112-738: Terrorist Networks in Pakistan and the Proliferation of IEDs,” 112th Cong. 2, December 13, 2013.

[16] Ibid.

[17] Daisy Muibu and Benjamin P. Nickels, “Foreign Technology or Local Expertise? Al-Shabaab’s IED Capability,” CTC Sentinel 10:10 (2017); “National Terrorism Advisory System Bulletin,” U.S. Department of Homeland Security, May 24, 2023.

[18] “Counterterrorism Guide: Al-Shabaab,” National Counterterrorism Center, 2014.

[19] Isel Ras and Halkano Wario, “Kenya’s Rise in Al-Shabaab attacks calls for localized solutions,” Institute for Security Studies, July 12, 2023; “Kenya: Al-Shabaab Attacks Surge Ahead of Somalia-Kenya Border Reopening,” Armed Conflict Location and Event Data Project (ACLED), July 7, 2023.

[20] “IED Attacks Continue to Take Their Toll on Civilians in Mali,” United Nations Multidimensional Integrated Stabilization Mission in Mali, April 20, 2023; Melissa Pavlik, Hilary Matfess, Heni Nsaibia, and Jules Duhamel, “Explosive Developments: The Growing Threat of IEDs in Western Niger,” Armed Conflict Location and Event Data (ACLED) Project, June 19, 2019; Elodie Hainard and David Lochhead, “A Primer: IEDs in the Sahel and West Africa,” Small Arms Survey, December 2, 2021.

[21] “Impact of Improvised Explosive Devices on Civilians in Afghanistan,” UNAMA Human Rights, May 30, 2023.

[22] Liz Throssell, ”Afghanistan: Impact of Improvised Explosive Devices on Civilians,” Office of the UN High Commissioner for Human Rights, June 27, 2023.

[23] Ibid.

[24] Andrew Mines, “The Evolving Terrorism Threat to the U.S. from the Afghanistan-Pakistan Region,” Program on Extremism and the National Counterterrorism Innovation, Technology, and Education Center (NCITE), August 2023.

[25] Bryan Burrough, Days of Rage: America’s Radical Underground, the FBI, and the Forgotten Age of Revolutionary Violence (New York: Penguin Books, 2016); Debbie Elliott and Taylor Jennings-Brown, “Her sister was killed in the Birmingham church bombing. A new book tells their story,” NPR: All Things Considered, September 14, 2022; “Wall Street Bombing 1920,” FBI, 2005.

[26] Audrey Kurth Cronin, Power to the People: How Open Technological Innovation Is Arming Tomorrow’s Terrorists (Oxford: Oxford University Press, 2019).

[27] Michael Balsamo, “A year after Jan. 6, FBI still hunting for pipe bomber and other insurrection suspects,” Associated Press, January 5, 2022.

[28] Robert Legare and Andres Triay, “20-year old Jordanian national living in Texas allegedly ‘trained with weapons to possibly commit an attack,’ feds say,” CBS News, October 31, 2023.

[29] Jeff Seldin, “Philadelphia Teenager Arrested in Terror Plot,” Voice of America News, August 14, 2023.

[30] Francisco Guzman, “FBI arrests Philadelphia teen, says he was talking to terrorists, buying bomb materials,” USA Today, August 15, 2023.

[31] “Teen Arrested in Federal Terrorism Probe to be Prosecuted by Philly DA,” Philadelphia District Attorney’s Office, August 14, 2023.

[32] “MLK Day Bomb Suspect Has Extensive White Supremacist Background,” Southern Poverty Law Center, June 17, 2011.

[33] “MLK Day Bomber: Horrific Hate Crime Prevented; Case Solved,” FBI, January 13, 2012; Peter W. Singer, “The Evolution of Improvised Explosive Devices (IEDs),” Brookings Institute, February 7, 2012.

[34] “FBI: Pipe Bomb Found on MLK Parade Route,” ABC News, January 18, 2011.

[35] “Ohio man sentenced to 14 years for plotting attack on Cleveland’s July 4th parade in an effort to support al Qaeda,” U.S. Department of Justice, February 11, 2020.

[36] Ibid.

[37] Tess Owns, “Aspiring Cleveland bomber planned to drive remote-controlled cars full of explosives into a Fourth of July parade,” VICE News, July 2, 2018.

[38] “3 men plead guilty to domestic terrorism crime related to plans to attack power grids,” U.S. Attorney’s Office, Southern District of Ohio, February 23, 2022.

[39] “Two Men Sentenced for Conspiring to Provide Material Support to Plot to Attack Power Grids in the United States,” U.S. Department of Justice, April 21, 2023; United States v. Collins et al., 7:20-cr-00167-M (E.D.N.C. August 18, 2021).

[40] “3 men plead guilty to domestic terrorism crime related to plans to attack power grids.”

[41] Jesus Jiménez, “3 Men Plead Guilty in Plot to Attack U.S. Power Grid,” New York Times, February 23, 2022.

[42] Ilana Krill and Bennett Clifford, “Mayhem, Murder, and Misdirection: Violent Extremist Attack Plots Against Critical Infrastructure in the United States, 2016-2022,” Program on Extremism, George Washington University, September 2022.

[43] Ted Lawson, “SAFECOM and NCSWIC Release Communications Dependencies Case Study: Nashville Christmas Day Bombing,” Cybersecurity and Infrastructure Security Agency, June 13, 2022.

[44] Elizabeth Clement-Webb, “FBI Releases Report on Nashville Bombing,” FBI Memphis Field Office, March 15, 2021.

[45] “Counterinsurgency Lessons Learned No. 53: Viet Cong Improvised Explosive Mines and Booby Traps,” United States Army, Army War College, 1966.

[46] Kirk Yeager, “Improvised explosives characteristics, detection, and analysis” in A. Beveridge and D. R. Gaskell eds., Forensic Investigation of Explosions (Boca Raton, FL: CRC Press, 2012), pp. 493-538.

[47] Brian A. Jackson and Dave Frelinger, “Emerging threats and security planning: how should we decide what hypothetical threats to worry about?” Rand Corporation, 2009.

[48] “Anarchist/Left-Wing Violent Extremism in America: Trends in Radicalization, Recruitment, and Mobilization,” Program on Extremism, George Washington University and National Counterterrorism Innovation, Technology, and Education Center (NCITE), November 2021; Jeff Breinholt, “Books as Contraband: The Strange Case of the ‘Anarchist Cookbook,’” War on the Rocks, September 13, 2018; Azmat Khan, “The Magazine that ‘Inspired’ the Boston Bombers,” Frontline, April 30, 2013; Matthew Kriner and Bjørn Ihler, “Analysing Terrorgram Publications: A New Digital Zine,” Global Network on Extremism and Technology, September 12, 2022.

[49] Paul Gill, John Horgan, and Paige Deckert, “Tracing the Motivations and Antecedent Behaviors of Lone-Actor Terrorism: A Routine Activity Analysis of Five Lone-Actor Terrorist Events,” International Center for the Study of Terrorism, August 2012.

[50] Brian Hughes and Cynthia Miller-Idriss, “Uniting for Total Collapse: The January 6 Boost to Accelerationism,” CTC Sentinel 14:4 (2021).

[51] C.J.M. Drake, “The Role of Ideology in Terrorists’ Target Selection,” Terrorism and Political Violence 10:2 (1998): pp. 53-85.

[52] Krill and Clifford.

[53] Michael Logan, “‘Ideology matters’: NCITE shares expertise at agriculture threats symposium,” National Counterterrorism Innovation, Technology, and Education Center (NCITE), June 26, 2023.

[54] Pete Simi and Seamus Hughes, “Understanding Threats to Public Officials,” National Counterterrorism Innovation, Technology, and Innovation Center, August 2023.

[55] “Massachusetts Man Arrested for Making Bomb Threat to Arizona State Election Official,” U.S. Department of Justice, July 29, 2022.

[56] “Man Pleads Guilty to Sending Bomb Threat to Arizona State Election Official,” U.S. Department of Justice, August 11, 2023.

[57] Pete Simi and Seamus Hughes, “Understanding Threats to Public Officials,” National Counterterrorism Innovation, Technology, and Education Center (NCITE), August 6, 2023; Don Rassler, “A View from the CT Foxhole: Ravi Satkalmi, Director of Intelligence, United States Capitol Police,” CTC Sentinel 16:7 (2023).

[58] Thomas P. Forbes, Shannon T. Krauss, and Greg Gillen, “Trace Detection and Chemical Analysis of Homemade Fuel-Oxidizer Mixture Explosives: Emerging Challenges and Perspectives,” Trends in Analytical Chemistry: TRAC 131 (2020).

[59] “Mission hall exhibit: Remembering the past, informing the future,”Transportation Security Authority, 2020.

[60] Paul Gill et al., “Terrorist Use of the Internet by the Numbers: Quantifying Behaviors, Patterns, and Processes,” Criminology and Public Policy 16:1 (2017); Brian Fishman, “Crossroads: Counter-terrorism and the Internet,” Texas National Security Review 2:2 (2019).

[61] Kirk Yaeger, “The Burden of Knowledge,” Propellants, Explosives, Pyrotechnics 39 (2014): pp. 159-160; “Al Qaeda Releases First-Ever English Terror Magazine,” Anti-Defamation League, July 15, 2010.

[62] “European Union Terrorism Situation and Trend Report,” Europol, Publications Office of the European Union, Luxembourg, 2023.

[63] Lorand Bodo and Inga Kristina Trauthig, “Emergent Technologies and Extremists: The DWeb as a New Internet Reality?” Global Network on Extremism and Technology (GNET), July 2022; Joel S. Elson, Austin C. Doctor, and Sam Hunter, “The metaverse offers a future full of potential – for terrorists and extremists, too,” Conversation, January 7, 2022.

[64] Marc Tranchemontagne, “The Enduring IED Problem: Why We Need Doctrine,” National Defense University Press, January 1, 2016.

[65] Austin C. Doctor and James Igoe Walsh, “The Militant Drone Playbook,” War on the Rocks, August 12, 2021; Austin C. Doctor and James I. Walsh, “The Coercive Logic of Militant Drone Use,” Parameters 51:2 (2021); Don Rassler, The Islamic State and Drones: Supply, Scale, and Future Threats (West Point, NY: Combating Terrorism Center, 2018).

[66] Austin Doctor, “The Militant Drone Threat is No Longer New. Why Does It Still Feel Novel?” Modern War Institute, West Point, February 24, 2022.

[67] Paul Cruickshank and Tim Lister, “Analysis: Model planes as weapons of terror,” CNN, September 29, 2011.

[68] “Man Sentenced in Boston for Plotting Attack on Pentagon and U.S. Capitol and Attempting to Provide Detonation Devices to Terrorists,” U.S. Department of Justice, November 1, 2012.

[69] “Man Sentenced in Boston for Plotting Attack on Pentagon and U.S. Capitol and Attempting to Provide Detonation Devices to Terrorists,” U.S. Attorney’s Office, November 1, 2012.

[70] Austin C. Doctor and James I. Walsh, “The Coercive Logic of Militant Drone Use,” Parameters 51:2 (2021); Stephen Kalin, “Behind Ukraine’s Deadly Drones: Putin’s Invasion and Biden’s Limits,” Wall Street Journal, August 2, 2023; Nick Waters, “Types of Islamic State Drone Bombs and Where to Find Them,” Bellingcat, May 24, 2017.

[71] “Senate Homeland Security Hearing on National Security Threats,” CSPAN, November 17, 2022; “FBI Probing Cases of Bomb-laden Drones in US,” AFP News, November 17, 2022.

[72] “Coventry student built drone to deliver a bomb for Islamic State, court told,” BBC News, August 23, 2023.

[73] Ibid.

[74] Brian Barrett, “A Drone Tried to Disrupt the Power Grid. It Won’t Be the Last,” Wired, November 5, 2021.

[75] T.X. Hammes, “Terror and Technology From Dynamite to Drones,” War on the Rocks, September 4, 2020; Cronin.

[76] ”First Responders Toolbox – Programmable Microcontrollers: Potential for Illicit Use,” NCTC, DHS, FBI, December 10, 2020.

[77] Ibid.

[78] ”How to Address the Harms from IEDs,” Action on Armed Violence, October 12, 2022.

[79] Jeff Stewart, “3D Printing of Energetic Materials and Components for Optimized or Enabling Technologies,” Innovation and Partnerships Office, Lawrence Livermore National Laboratory, Livermore, California, 2017.

[80] “Maryland Woman and Florida Man Charged Federally for Conspiring to Destroy Energy Facilities,” U.S. Department of Justice, February 6, 2023; Lea Skene and Eric Tucker, “Woman plotted with neo-Nazi to attack power grid, feds say,” Associated Press, February 6, 2023.

[81] “Summary of the Terrorism-Related Threat to the United States,” National Terrorism Advisory System, U.S. Department of Homeland Security, May 24, 2023.

[82] Walter Laqueur, A History of Terrorism (New York: Routledge, 2001).

[83] Pete Simi and Seamus Hughes, “By the Numbers: Understanding threats to public officials,” National Counterterrorism Innovation, Technology, and Education Center (NCITE), August 6, 2023; Nicole Sganga, “FBI and DHS warn of increased threats to law enforcement and government officials after Mar-a-Lago search,” CBS News, August 15, 2022.

[84] Bruce Hoffman and Jacob Ware, “The Accelerating Threat of the Political Assassination,” War on the Rocks, August 24, 2022.

[85] “Venezuela President Maduro survives ‘drone assassination attempt,’” BBC, August 5, 2018.

[86] Nick Waters, “Did Drones Attack Maduro in Caracas?” Bellingcat, August 7, 2018.

[87] David Kilcullen, Out of the Mountains: The Coming Age of the Urban Guerrilla (New York: Hurst Publishers, 2015).

[88] “Homeland Security Presidential Directive/HSPD-19,” The White House, February 12, 2007.

[89] Austin Doctor, “N-SIGHTS: 2023 Terrorism Outlook,” National Counterterrorism Innovation, Technology, and Education Center (NCITE), January 31, 2023.

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