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What NIJ Research Tells Us About Domestic Terrorism

NIJ-funded research projects have led to a better understanding of the processes that result in violent action, factors that increase the risk of radicalizing to violence, and how best to prevent and respond to violent extremism.
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National Institute of Justice Journal
Date Published
January 4, 2024

Militant, nationalistic, white supremacist violent extremism has increased in the United States. In fact, the number of far-right attacks continues to outpace all other types of terrorism and domestic violent extremism. Since 1990, far-right extremists have committed far more ideologically motivated homicides than far-left or radical Islamist extremists, including 227 events that took more than 520 lives.[1] In this same period, far-left extremists committed 42 ideologically motivated attacks that took 78 lives.[2] A recent threat assessment by the U.S. Department of Homeland Security concluded that domestic violent extremists are an acute threat and highlighted a probability that COVID-19 pandemic-related stressors, long-standing ideological grievances related to immigration, and narratives surrounding electoral fraud will continue to serve as a justification for violent actions.[3]

Over the past 20 years, the body of research that examines terrorism and domestic violent extremism has grown exponentially. Studies have looked at the similarities and differences between radicalization to violent domestic ideologies and radicalization to foreign extremist ideologies. Research has found that radicalization processes and outcomes — and perhaps potential prevention and intervention points — vary by group structure and crime type. In addition, research has explored promising and effective approaches for how communities can respond to radicalization and prevent future attacks.[4]

The National Institute of Justice (NIJ) has played a unique role in the evolving literature on terrorism and violent extremism. NIJ has promoted the development of comprehensive terrorism databases to help inform criminal justice responses to terrorism, address the risk of terrorism to potential targets, examine the links between terrorism and other crimes, and study the organizational, structural, and cultural dynamics of terrorism. In 2012, the U.S. Congress requested that NIJ build on these focal points by funding “research targeted toward developing a better understanding of the domestic radicalization phenomenon and advancing evidence-based strategies for effective intervention and prevention.”[5] NIJ has since funded more than 50 research projects on domestic radicalization, which have led to a better understanding of the processes that result in violent action, factors that increase the risk of radicalizing to violence, and how best to prevent and respond to violent extremism.

This article discusses the findings of several NIJ-supported domestic radicalization studies that cover a range of individual and network-centered risk and protective factors that affect radicalization processes, including military involvement and online environments. The article also explores factors that shape the longevity of radicalization processes and their variation by group structure and crime type, and examines factors that affect pathways away from domestic extremism. It concludes with a discussion of how these findings can inform terrorism prevention strategies, criminal justice policy, and community-based prevention programming.

The Characteristics of U.S. Extremists and Individuals Who Commit Hate Crimes

Over the past two decades, research that seeks to understand individual-level engagement in violent extremism has grown tremendously. However, as the research field has developed, a gap has emerged between the increasingly sophisticated arguments that scholars use to explain extremism and the availability of data to test, refine, and validate theories of radicalization.

In 2012, NIJ funded the Empirical Assessment of Domestic Radicalization project to address the data gap in radicalization research.[6] The project created the Profiles of Individual Radicalization in the United States (PIRUS) database, a cross-ideological repository of information on the characteristics of U.S. extremists. In 2017, NIJ supported a follow-on project[7] that sought to replicate the PIRUS data for individuals in the United States who commit hate crimes. This project yielded the Bias Incidents and Actors Study (BIAS) dataset, the first data resource for researchers and practitioners interested in understanding the risk and protective factors associated with committing hate crimes.

PIRUS and BIAS are designed to provide users with information on a wide range of factors that can play a role in a person’s radicalization to criminal activity.[8] These risk and protective factors can be divided into four domains:[9]

  • The situational characteristics of the crimes, including whether the acts were premeditated or spontaneous, involved co-conspirators, or were committed while under the influence of drugs and alcohol.
  • The characteristics of the victims, including whether targets were “hard” (for example, military bases, secure facilities) or “soft” (for example, businesses, public areas, private civilians) and whether the individuals had prior relationships with their victims.
  • Factors that produce the social bonds that may protect against mobilization to violence, such as marriage, military service, work experience, and advanced education.
  • Factors that may act as radicalization mechanisms and risk factors for violence, such as previous criminal activity, membership in extremist or hate groups, substance use, and mental illness.

The PIRUS and BIAS data have been used to generate insights on a range of important topics related to hate crime and extremism; however, there are three overarching findings common to both datasets: diversity in beliefs, diversity in behaviors, and diversity in characteristics.

Diversity in Beliefs

Although it is not uncommon for a particular ideology to dominate the public discourse around extremism, the PIRUS and BIAS data indicate that U.S. extremists and individuals who commit hate crimes routinely come from across the ideological spectrum, including far-right, far-left, Islamist, or single-issue ideologies. These ideologies break down into particular movements, or sub-ideologies. For instance, in 2018, the PIRUS data identified extremists associated with several anti-government movements, Second Amendment militias, the sovereign citizen movement, white supremacy, ecoterrorism, anarchism, the anti-abortion movement, the QAnon conspiracy theory, and others.[10] The prevalence of particular movements can ebb and flow over time depending on political climate and law enforcement priorities, but at no point in recent U.S. history has one set of beliefs completely dominated extremism or hate crime activity.[11] Furthermore, the PIRUS and BIAS data reveal that U.S. extremists and individuals who commit hate crimes are often motivated by overlapping views. For instance, it is common for individuals from the anti-government militia movement to adopt views of white supremacy or for those from the extremist environmental movement to take part in anarchist violence. Nearly 17% of the individuals in PIRUS were affiliated with more than one extremist group or sub-ideological movement, and nearly 15% of the individuals in BIAS selected the victims of their hate crimes because of multiple identity characteristics, such as race and sexual orientation.[12]

Diversity in Behaviors

Although radicalization to violence has been a primary topic in extremism and hate crime research, the PIRUS and BIAS data indicate that U.S. extremists and individuals who commit hate crimes often engage in a range of violent and nonviolent criminal activities. Indeed, 42% of PIRUS and nearly 30% of BIAS individual actors engaged exclusively in nonviolent crimes, such as property damage, financial schemes, and illegal demonstrations.[13] Moreover, the violent outcomes represented in the PIRUS and BIAS data vary in scope and type. For instance, approximately 15% of those in BIAS committed or planned to commit mass casualty crimes, while the remaining subjects targeted specific victims.[14] Similarly, nearly 50% of those in BIAS did not premeditate their crimes but rather acted spontaneously after chance encounters with their victims.[15]

Diversity in Characteristics

One of the more common conclusions of recent research on radicalization is that no single profile accurately captures the characteristics of the individuals who commit extremist and hate crimes.[16] The PIRUS and BIAS data support this finding, revealing that background characteristics vary considerably depending on ideological affiliations. For instance, white supremacists in PIRUS tend to be older and less well-educated and are more likely to have criminal histories than those who were inspired by foreign terrorist groups, such as al-Qaida or the Islamic State of Iraq and Syria, or those associated with the extremist environmental or anarchist movements.[17] Despite these differences, some risk and protective factors tend to separate violent from nonviolent individuals, regardless of ideology.[18] In the PIRUS data, individuals with criminal records, documented or suspected mental illness, and membership in extremist cliques are more often classified as violent, while those who are married with stable employment backgrounds are more likely to engage in nonviolent crimes.[19] Similarly, in BIAS, violent individuals are more likely to co-offend with peers, have criminal histories that include acts of violence, and offend while under the influence of drugs or alcohol.[20]

Military Experience and Domestic Violent Extremism

According to current statistics, individuals with military backgrounds represent 11.5% of the total known extremists who have committed violent and nonviolent crimes in the United States since 1990.[21] Although this percentage seems small, there has been a growing trend of (former) military members engaging in extremist offenses in recent years. An average of seven people with U.S. military backgrounds per year committed extremist crimes between 1990 and 2010. That rate has risen to an average of 29 people per year over the past decade. Also worth noting is that more than half (52%) of extremists with military experience are identified as violent.

Given the growth of violent domestic extremism among military personnel, the relationship between military service and radicalization has become a major concern. Prior NIJ-funded studies have identified military experience as a potential risk factor for attempted and actual terrorism.[22] The likelihood of radicalization and radicalization to violence increases when individuals have already left military service.[23] This research suggests that military service is not a social bond that inhibits extremist violence.

NIJ studies have also shown that individuals with military experience may be susceptible to recruitment by domestic violent extremist groups due to their unique skills, which an extremist group may perceive as contributing to the success of a terrorist attack.[24] Also, transitioning from military to civilian life appears to be a pull factor for engaging in violent extremism.[25] Indicators for potential involvement in extremism may include a lack of a sense of community, purpose, and belonging. If these indicators are identified early, community stakeholders — in partnership with military agencies — could have an opportunity to intervene. Although such knowledge is valuable, the role of military service in radicalization to violent extremism still requires study.

Differences in Violent Extremist Characteristics Between Military Veterans and Civilians

In 2019, NIJ funded researchers at the University of Southern California to study the link between military service and violent domestic extremism. They are also examining the differences between military veteran and civilian extremists in terms of their characteristics and social networks.[26] Although this study is ongoing, preliminary findings have been drawn from a secondary analysis of the American Terrorism Study data, which contain information on people federally indicted for terrorism-related crimes by the U.S. government between 1980 and 2002.[27] With these data, the researchers compared the demographic and homegrown violent extremist characteristics among military veterans and civilians. The demographic characteristics considered were age, race, sex, marital status, and education level. The homegrown violent extremist characteristics consisted of the length of group membership, type of terrorist group, role in the group, mode of recruitment into the group, primary target, and the state of indictment.

The research team observed significant differences between military veteran and civilian extremists across both demographic and homegrown violent extremist characteristics. First, they found that military veteran and civilian extremists differed with respect to age, sex, and marital status. Specifically, individuals with military service who engaged in homegrown violent extremism were more likely to be older, male, and in marital or cohabiting relationships than civilians who engaged in homegrown violent extremism. Second, analyses revealed that, compared to civilian extremists, military veteran extremists had greater affiliations with right-wing terrorist groups (versus left-wing, international, or other terrorist groups) and were more likely to hold leadership positions within these groups and either initiate a terrorist group or unite groups together. Finally, other than government/federal officials or buildings, which were the primary targets across all groups, the primary targets of veterans were diverse social groups, such as those belonging to racial, ethnic, and religious minority groups.

Implications of Transitioning Out of Military Service

The University of Southern California researchers intend to supplement these results by interviewing members from the social networks of military veterans and civilians who committed homegrown violent extremism between 2003 and 2019. The findings produced thus far are important, especially because the association between military experience and terrorism is understudied. Ultimately, these results suggest that people who transition from active duty to veteran status experience a nuanced, complex, and potentially lifelong process. Veterans who encounter difficulties during this transition and desire — but lack — a sense of community, purpose, and belonging after leaving the military may be attracted to the pull of domestic extremist groups. In these groups, veterans can lead and collaborate with others of similar ideologies to accomplish a shared mission akin to what they did in the military. For example, the military veterans in this study largely endorsed right-wing values; thus, perhaps something about the narratives of right-wing extremist groups compensates for the void felt when leaving military service. With such insights in mind, researchers recommend forming partnerships among civilians, the military, and veteran communities to identify and prevent violent extremism among U.S. veterans.

Longevity of Terrorist Plots in the United States

A major question for researchers and counterterrorism officials is how to prevent the next act of terrorism or violent extremism from occurring. As such, much attention has been paid to disrupted plots and successful interdiction tactics that ultimately led to arrest and indictment. Less attention has been given to what those responsible for acts of terrorism and violent extremism do to successfully evade detection and arrest. In other words, the focus has not been on what terrorists and violent extremists are doing “right.”

In 2013, NIJ funded researchers at the University of Arkansas’ Terrorism Research Center to study the sequencing of precursor behaviors for individuals who have been federally indicted in the United States for charges related to terrorism and domestic violent extremism.[28] Based on preliminary analyses, the researchers somewhat serendipitously observed lifespan differences between lone actors and those operating in small cells or more formalized groups. Consequently, it warranted a more comprehensive examination of the factors that increased the likelihood of terrorists and violent extremists evading arrest. NIJ funded the researchers to identify behaviors that improved the chances of plot longevity — or the ability for terrorists to commit acts of terrorism and evade capture by law enforcement — for individuals federally indicted on terrorism-related charges.[29]

Data on the longevity of terrorism and violent extremism plots come from the American Terrorism Study, the longest-running project on terrorism and violent extremism in the United States. With NIJ funding that began in 2003,[30] the American Terrorism Study maintains the most comprehensive dataset on temporally linked precursor behaviors and outcomes of terrorism and violent extremism plots. To examine plot longevity, the Arkansas researchers[31] limited their analyses to 346 federally indicted individuals who were linked to the planning or completion of a terrorist attack in the United States from 1980 to 2015. Longevity, or duration of their “terrorist lifespan,” is based on the date of a person’s involvement in their first preparatory activity and their “neutralizing” date (usually the date of arrest).

One of the key findings from this research is a correlation between significant declines in the lifespan of individual terrorists and major changes to the U.S. Attorney General guidelines established to combat terrorism and violent extremism in the United States. For example, those who began in the mid- to late 1970s, following Watergate, COINTELPRO, and the Privacy Act, had a median longevity of 2,230 days. In contrast, the median lifespan of terrorists who began operating in the mid-1980s decreased to 1,067 days. Later, in the early 2000s, it fell even further to 99 days, which reflects the FBI’s tighter focus on terrorism and violent extremism and guidelines granting law enforcement more discretion in the investigative techniques employed.

The researchers also found that the lifespans of terrorists and violent extremists vary significantly depending on key attributes, such as ideology, sex, and educational attainment. For example, environmental and extreme left-wing violent extremists tend to sustain themselves for relatively long periods of time (5.4 and 4.3 years, respectively), while the longevity of extreme right-wing and radical Islamist terrorists is, on average, two years or less.

Females federally indicted on charges related to terrorism and violent extremism also tend to have increased longevity compared to male terrorists and violent extremists, perhaps because of females’ disproportionate representation in longer-lasting extreme left-wing and environmental movements, as well as increased representation in left-wing group leadership roles. Females involved in terrorism and extremism are usually more educated, which is also associated with extended longevity. Further, females who play support roles in terrorism and extremist groups — as is more often the case for right-wing extremists and radical Islamist terrorists — also appear to have longer lifespans. In contrast, males have been more likely to engage in overtly criminal preparatory behavior and actual incident participation than females. Both types of behavior are significantly more likely to attract the attention of law enforcement and would be expected to shorten the longevity of both male and female terrorists and violent extremists.

Finally, longevity also depends on a plot’s sophistication and the extent of the planning required to carry it out. Less sophisticated plans or executed plots, or those using simpler and less advanced weapons, are generally associated with longer lifespans for terrorists and violent extremists. More sophisticated plots may provide greater potential for missteps by terrorists and violent extremists and leads for law enforcement. Additionally, more sophisticated plots are associated with more meetings with accomplices and necessitate extra preparation. Importantly, both the number of meetings and preparatory activities have been found to be negatively related to the successful completion of terrorist incidents, suggesting that early intervention or arrest are also linked to these two factors.

How Domestic Terrorists Use the Internet

Terrorists and terrorist groups use the internet to share propaganda and recruit new members. The internet provides a platform to strengthen their members’ commitment to the cause, encourage radicalized individuals to act, and coordinate legal and illegal activities. A recently published meta-analysis concluded, “Exposure to radical content online appears to have a larger relationship with radicalization than other media-related risk factors (for example, television usage, media exposure), and the impact of this relationship is most pronounced for the behavioral outcomes of radicalization.”[32]

In 2014, NIJ funded a study to develop a deeper understanding of what domestic terrorists discuss on the internet.[33] The study analyzed 18,120 posts from seven online web forums by and for individuals interested in the ideological far right. The research team read each post’s content and coded it for either quantitative or qualitative analyses depending on the project’s objective.

The project provided several important insights into terrorist use of the internet. First, the web forums included discussions about a variety of beliefs, such as gun rights, conspiracy theories, hate-based sentiments, and anti-government beliefs; however, the intensity of ideological expression was generally weak. The nature of the online environments that far-right groups use likely facilitates the diffusion of ideological agendas.

Second, the amount and type of involvement in these forums played a key role in radicalization. Posting behaviors changed over time. Users grew more ideological and radical as other users reinforced their ideas and connected their ideas to those from other forums. (It is important to note that the study focused on online expression and not conversion to offline violence.)

Third, far-right extremists were primarily interested in general technology issues. Discussions focused on encryption tools and methods (such as Tor), internet service providers and social media platforms, and law enforcement actions to surveil illicit activities online. These far-right extremists appeared more interested in defensive actions than sophisticated schemes for radicalization or offensive actions such as criminal cyberattacks.

The study used social network analyses to visualize user communications and network connections, focusing on individuals’ responses to posts made within threads to highlight interconnected associations between actors. The social network analyses indicated that far-right forums have a low network density, which suggests a degree of information recycling between key actors. The redundant connections between actors may slow the spread of new information. As a result, such forums may inefficiently distribute new knowledge due to their relatively insular nature. They may also be generally difficult to disrupt, as the participants’ language and behaviors reinforce others and create an echo chamber. These networks are similar to others observed in computer hacker communities and data theft forums,[34] which suggests that there may be consistencies in the nature of online dialogue regardless of the content.

The study also indicated that extreme external events usually did not affect posting behaviors. However, there were significant differences associated with conspiratorial, anti-Islamic, and anti-immigrant posts after the Boston Marathon bombing. It may be that violence or major disruptive events inspired by jihadist ideologies draw great responses from far-right groups relative to their own actions. The same appears to be true for the 2012 presidential election; the study observed increases both in the number of posts in the month after the election and in overt signs of individual ties or associations to far-right movements through self-claim posts, movement-related signatures, and usernames. These findings are consistent with other recent work comparing online mobilization after the 2012 and 2016 presidential elections.[35]

Entering and Exiting White Supremacy in the United States

An NIJ-funded research team led by RTI International examined the complex social-psychological processes involved with entering, mobilizing, and exiting white supremacy in the United States.[36] The researchers conducted in-depth life history interviews with 47 former members of white supremacist groups in 24 states and two provinces in Canada.[37]

For this project, white supremacy referred to groups that reject essential democratic ideals, equality, and tolerance. A key organizing principle is that inherent differences between races and ethnicities position white and European ancestry above all others. Those interviewed were authoritarian, anti-liberal, or militant nationalists who had a general intolerance toward people of color. They had used violence to achieve their goals and supported a race war to eradicate the world of nonwhite people.[38]

The study led to several key findings about entering and exiting white supremacy in the United States.

Hate as Outcome

The study found that most people do not join white supremacist groups because they are adherents of a particular ideology. Rather, a combination of background factors increases the likelihood that someone will be susceptible to recruitment messaging (for example, propaganda).[39] Previous research has highlighted that hate or adherence to racist violence was an outcome of participation in white supremacist groups.[40] The commitment to white supremacist groups lacked a preexisting sense of racial grievance or hatred that motivated an individual to join the racist movement.[41] One former member reported having “no inkling of what [Nazism] really was other than what you saw on TV.”[42] The NIJ-funded study found that people joined white supremacist groups because they were angry, lonely, and isolated, and they were looking for opportunities to express their rage.[43]

Vulnerabilities as Precondition

The former white supremacists had various personal, psychological, and social vulnerabilities that made them strive for what psychologists have framed as developing a new possible self.[44] High levels of negative life experiences — including, but not limited to, maladjustment, abuse, and family instability — potentially make a person imagine a new, different, and more fulfilled self.[45] They can imagine an empowered future self with friends and a purpose. Extremist recruiters prey on these desires. The former white supremacists indicated high levels of physical, sexual, and psychological abuse as children; strained personal relationships; and general difficulties throughout their lives. These struggles made white supremacy seem like an improvement to their sense of self, as the group came with a ready-made set of friends, social events, and camaraderie among individuals with similarly rough pasts. Besides these social benefits, white supremacist groups provided members with a deeper sense of belonging and explanation for their life troubles, rooted in a sense of racial pride and empowerment.

Gradual, Nonlinear Exit

Most white supremacists in this country do not remain members for life. Rather, group membership is often temporary (but not always short-lived), and many become disillusioned and burnt out over time. The study showed that the exit process is gradual, as the former white supremacists reported slowly becoming dissatisfied with the ideology, tactics, or politics of a group.[46] They described an identity that became filled with negative encounters with other members, even breeding distrust. White supremacy requires the development of a totalizing identity that results in isolating members from nonextremists. This marginalization fosters a sense of social stigma that makes white supremacy less attractive and further supports disengagement and deradicalization processes.

This research reported that emotional dynamics create trajectories of development and decline in white supremacy and the role of disillusionment among the reasons why members exit the organization.[47] These analyses offer an explanation for how white supremacist organizations maintain solidarity even though many individuals stay in groups after losing their ideological commitment. They also demonstrate that exit from a group is a nonlinear process.[48] Meanwhile, in other analyses, the study team reported that, even after an individual exits a group, their white supremacist identity lingers with a residual effect.[49] That research likened hate to an addiction that creates an uncontrollable emotional, social, and cognitive hold over adherents, which has the ability to pull former members back into hate almost against their will.[50] The former white supremacists shared experiences in which music, environments, and images created desire, longing, and curiosity about their old lifestyle within the organization.


The NIJ-funded study found several blind spots in terms of identification and awareness among criminal legal system practitioners and other responders. This resulted in several missed opportunities for intervention and practical solutions. Exhibit 1 details four areas in which the study findings can contribute to criminal justice policy and practice.[51]

Exhibit 1. Missed Opportunities for Intervention and Practical Solutions

Policy Implications

The results of the NIJ-funded studies discussed in this article have several implications for policy and practice. First, they illustrate that extremism is complex and that successfully countering it will require a unified response that bridges law enforcement, community partners, health officials, and concerned citizens. To facilitate a shared understanding of the extremist threat, stakeholders engaged in counterextremism efforts routinely use findings from these studies to provide training to concerned family and friends about potential radicalization warning signs and how best to respond. They also use the findings to educate law enforcement, corrections and probation officers, and mental health professionals on the complexity of radicalization so they can accurately gauge and respond to extremism in their communities. These types of training initiatives will remain critical to counterextremism efforts as the threat continues to evolve.

Second, the studies highlight the importance of focusing criminal justice resources on domestic extremism. Although international terrorist organizations remain a threat, these studies show that domestic extremists continue to be responsible for most terrorist attacks in the United States. Historically, far fewer resources have been dedicated to the study of domestic extremism, leaving gaps in our understanding about terrorist trends, recruitment and retention processes, and online behaviors. Due in large part to NIJ’s commitment to funding research on domestic radicalization, considerable progress has recently been made in addressing these topics. But this work will need to continue if we hope to keep pace with the rapidly evolving threat landscape.

Finally, the studies highlight the need for communitywide partnerships that link government and nongovernment organizations in support of community-level prevention and intervention programs. Law enforcement and criminal justice resources for countering extremism are finite and scarce, making it imperative that we focus our research and support efforts on understanding what occurs before a crime takes place. As the studies reviewed in this article show, there is often an opportunity to intervene to help individuals exit extremism before they engage in criminal activity. Similarly, prevention efforts are needed in digital spaces where extremist narratives often flourish. Achieving these goals will require community members, policymakers, and practitioners to commit to supporting counterextremism efforts.

About This Article

This article was published as part of NIJ Journal issue number 285. This article discusses the following awards:

Opinions or points of view expressed in this document represent a consensus of the authors and do not necessarily represent the official position, policies, terminology, or posture of the U.S. Department of Justice on domestic violent extremism. The content is not intended to create, does not create, and may not be relied upon to create any rights, substantive or procedural, enforceable at law by any party in any matter civil or criminal.

Date Published: January 4, 2024