By Jillian Peterson, ; Gina Erickson; Kyle Knapp, James Densley

Understanding the motivation of a mass shooter’s intent to do harm can help practitioners and policy makers develop more effective intervention strategies. OBJECTIVE To examine the prevalence of communication of intent to do harm, known as leakage, in a sample of 170 mass public shooters from 1966 to 2019; the characteristics of perpetrators who do and do not leak their plans; and whether leakage is a form of fame-seeking behavior or a cry for help among individuals who are in crisis or suicidal. DESIGN, SETTING, AND PARTICIPANTS This cross-sectional study included perpetrators who killed 4 or more people in a public space from 1996 to 2019 and were included in a comprehensive database of US mass shootings. That database was built from August 2017 to December 2019, and analysis took place from January to May 2021. MAIN OUTCOMES AND MEASURES Leakage was identified and coded using publicly available records. Any communication to a third party of an intent to do harm prior to the shooting was coded as leakage. Logistic regression models were used to examine the risk factors associated with leakage. Models estimating leakage were examined to assess the 2 hypothesized pathways to leakage (the cry-for-help model and the fame-seeking model). RESULTS The 170 participants in this sample included 166 (97.7%) male perpetrators and 3 (2.3%) female perpetrators, with a mean (SD) age of 34 (12) years. Overall, 161 participants had known race and ethnicity: 11 (6.8%) Asian individuals, 35 (21.7%) Black individuals, 14 (8.7%) Latinx individuals, 7 (4.4%) Middle Eastern individuals, 3 (1.9%) Native American individuals, 89 (55.3%) White individuals, and 2 (1.2%) individuals with other race and ethnicity. Overall, 79 mass shooters (46.5%) leaked their plans. Of perpetrators who leaked their plans, 35 (44.3%) leaked specific plans about a mass shooting, and 44 (55.1%) leaked nonspecific plans about generalized violence. The study findings indicate that leakage was associated with receiving counseling (odds ratio, 7.0; 95% CI, 2.0-24.8) and suicidality (odds ratio, 3.7; 95% CI, 1.0-13.6), suggesting that leakage may best be characterized as a cry for help from perpetrators prior to their act. CONCLUSIONS AND RELEVANCE In this study, nearly half of the perpetrators of mass shootings leaked their plans. Leakage was associated with receiving counseling and suicidality. Leakage is a critical moment for mental health intervention to prevent gun violence. Opportunities to report threats of violence need to be increased. Traditional threat assessment models focused on specific threats of violence may miss critical opportunities for intervention. 

JAMA Network Open. 2021, 9p.

By Alex Nowraster

A total of 298 mass shooters were responsible for 1,733 murders and 2,459 people injured in the United States from 1966 through the end of 2024. Over the 59 years studied here, the chance of being murdered by a mass shooter was about 1 in 9.1 million per year, and the annual chance of being injured was about 1 in 6.4 million. The murder victims of mass shooters account for about 0.15 percent of all homicides from 1966 through the end of 2024, but that percentage has risen in recent years. The deadliest year for mass shootings was 2017, when 14 shooters murdered 130 people and injured 959 others, accounting for 0.7 percent of all homicides in that year. Of those 298 shooters, 255 were native-born and 43 were foreign-born. The chance of being murdered in a mass shooting committed by a native-born American was about 1 in 10.5 million per year, about 6.5 times higher than the chance of being killed by a foreign-born mass shooter, which was about 1 in 68.4 million per year. The annual chance of being injured by a foreign-born mass shooter was about 1 in 94.7 million, and the chance of being injured by a native-born shooter was about 1 in 6.9 million per year. About 14.4 percent of mass shooters were foreign born, and they were responsible for 13.3 percent of murders, roughly in line with their share of the population, and 6.8 percent of injuries in mass shootings, roughly half their share of the population.

Washington, DC: Cato Institute, 2025. 10p.

By Jason R. Silva

This study examines the reality and news media coverage of all mass shootings in the United States from 1966 to 2016. It employs agenda-setting and framing theoretical frameworks to determine the social construction of mass shootings via the mass media. The project uses open-source data to create a comprehensive list of mass shooting incidents. It then identifies all published New York Times articles on each incident. The study summarizes both the reality of the social problem (i.e. incidents) and the news mediated reality (i.e. New York Times). Next, this dissertation conducts a media distortion analysis to determine the perpetrator, motivation, and incident characteristics influencing media selection, prominence, and framing. The purpose is to illustrate the media’s social construction of mass shootings that in turn shapes public perceptions, political discourse, and public policies. The study concludes by highlighting the findings and implications for scholars, practitioners, policy-makers, media outlets, and the general public.

New York: City University of New York (CUNY), 2019. 164p

By Jillian Peterson; Gina Erickson; Kyle Knapp; and James Densley

Understanding the motivation of a mass shooter’s intent to do harm can help practitioners and policy makers develop more effective intervention strategies. OBJECTIVE To examine the prevalence of communication of intent to do harm, known as leakage, in a sample of 170 mass public shooters from 1966 to 2019; the characteristics of perpetrators who do and do not leak their plans; and whether leakage is a form of fame-seeking behavior or a cry for help among individuals who are in crisis or suicidal. DESIGN, SETTING, AND PARTICIPANTS This cross-sectional study included perpetrators who killed 4 or more people in a public space from 1996 to 2019 and were included in a comprehensive database of US mass shootings. That database was built from August 2017 to December 2019, and analysis took place from January to May 2021. MAIN OUTCOMES AND MEASURES Leakage was identified and coded using publicly available records. Any communication to a third party of an intent to do harm prior to the shooting was coded as leakage. Logistic regression models were used to examine the risk factors associated with leakage. Models estimating leakage were examined to assess the 2 hypothesized pathways to leakage (the cry-for-help model and the fame-seeking model). RESULTS The 170 participants in this sample included 166 (97.7%) male perpetrators and 3 (2.3%) female perpetrators, with a mean (SD) age of 34 (12) years. Overall, 161 participants had known race and ethnicity: 11 (6.8%) Asian individuals, 35 (21.7%) Black individuals, 14 (8.7%) Latinx individuals, 7 (4.4%) Middle Eastern individuals, 3 (1.9%) Native American individuals, 89 (55.3%) White individuals, and 2 (1.2%) individuals with other race and ethnicity. Overall, 79 mass shooters (46.5%) leaked their plans. Of perpetrators who leaked their plans, 35 (44.3%) leaked specific plans about a mass shooting, and 44 (55.1%) leaked nonspecific plans about generalized violence. The study findings indicate that leakage was associated with receiving counseling (odds ratio, 7.0; 95% CI, 2.0-24.8) and suicidality (odds ratio, 3.7; 95% CI, 1.0-13.6), suggesting that leakage may best be characterized as a cry for help from perpetrators prior to their act.  

JAMA Netw Open. 2021;4(11):e2133073. doi:10.1001/jamanetworkopen.2021.33073

By Leslie M. Barnard; Erin Wright-Kelly; Ashley Brooks-Russell; et al

  Mass shootings (MS) account for less than 1% of firearm deaths in the US, but the frequency has increased.1 Risk factors for MS perpetration include societal discrimination, contagion effects, firearm access, mental illness, and substance abuse.2 Previous geographically and analytically limited studies found MS with handguns had higher fatality rates than those with rifles,3 and following an age-based assault weapons (AWs) restriction there was a reduction in firearm violence from AWs.4 Another study found that the 1994 federal AWs ban was associated with fewer MS.5 To further investigate the association between type of firearm and lethality of MS, this study examined what firearms were present at publicly targeted fatal MS and determined if AWs were associated with a higher number of injuries or deaths.

JAMA Netw Open. 2025;8(2):e2458085. doi:10.1001/jamanetworkopen.2024.58085 4p.

By The Violence Policy Center

Large capacity ammunition magazines are the common thread running through most mass shootings in the United States. Since 1980, there have been at least 106 mass shootings (three or more fatalities not including the shooter) where the shooter used large capacity ammunition magazines containing more than 10 rounds. A total of 959 people were killed in these shootings, and 1,309 were wounded. This number is likely a significant undercount of actual incidents as there is no consistent collection or reporting on this data. Even in many high-profile shootings, information on magazine capacity is neither released nor reported. The Violence Policy Center has compiled this list of incidents by analyzing news reports and follow-up investigative reports on mass shootings. Only shootings in which there is specific information that large capacity magazines were used are included.  ( This document was last updated on January 11, 2024.

Washington, DC: Violence Policy Center, 2024. 17p. 

By Rachel A. Kane; Basia E. Lopez; Paul A. Haskins

This Special Report of the National Institute of Justice (NIJ) presents a synthesis of select findings from over 60 publications on 18 NIJ-supported research projects on public mass shootings, including school mass shootings, since 2014. It also identifies areas of need and interest for future research and recommendations. For the purpose of this report, we define public mass shooting as a shooting event in a public setting that causes at least four fatalities in a single event. Excluded from our definition are events that occur in the course of a separate felony or domestic incident. However, the definition of a mass shooting varies among sources; for more information see “The Challenges of Defining ‘Mass Shooting’” in Part I of this report.

Washington, DC: U.S. Department of Justice Office of Justice Programs National Institute of Justice , 2023. 42p.

By Brent R. Klein,

Joshua D. Freilich and

Steven M. Chermak

The American School Shooting Study (TASSS) is an ongoing mixedmethod project funded by the National Institute of Justice to catalog US school shootings. It has amassed data based on open sources and other public materials dating back to 1990. This brief presents new insights from TASSS, diving deeper into the database’s potential to examine the locations, timing, and student involvement of youth-perpetrated gun violence.

Although statistically rare, fatal and nonfatal shootings in the United States at elementary, middle, and secondary schools remain important crime problems with significant public policy implications. Indeed, the impact of such violence exceeds the devastating fatalities and immense sorrow that survivors, families, and communities experience. Even one gunshot fired at a school can subject numerous individuals to the traumas of gun violence.1 Recent polls indicate that school shootings can also sway broader public views on crime, including attitudes toward violence reduction.2 As a result, school shootings have become a focal point of US politics, sparking crucial debates on the most effective strategies for preventing and responding to gun violence, both inside and outside K-12 schools.

Albany, NY: Rockefeller Institute of Government, 2023. 21p.

By Sarah Watson

Mass sho are otingsone example of a focusing event that has particular significance for firearms legislation. Mass shootings shock, disturb and provoke enormous and controversial debate, often causing significant public and media resonance, becoming the subject of intense discussion politically. At times providing an impetus for legislative amendments, often in distinct ways that routine gun violence does not. If certain events highlight the need for reform, policy change becomes more likely. Cases with the lowest number of victims are likely to generate the least amount of attention and are most likely to be missed in data collection, rendering them the least noteworthy, least important in terms of lethality and social and political consequence. Various problems come to the attention of people in and around government, necessitating an understanding of why such problems occupy officials’ attention and appear to be more ‘deserved>

Criminology & Criminal JusticeOnlineFirst, July 19, 2022

By Jaclyn Schildkraut and H. Jaymi Elsass

An Updated Look at Mass Public Shootings in America Over the last two years, there have been a number of high-profile mass public shootings in the United States that have dominated the headlines and captured the attention of the public and policymakers alike—from Buffalo, New York (May 14, 2022; 10 dead) to Uvalde, Texas (May 24, 2022; 21 dead), Highland Park, Illinois (July 4, 2022; 7 dead), Monterrey Park, California (January 21, 2023; 11 dead), and Louisville, Kentucky (April 10, 2023; 5 dead). Although mass public shootings rarely made headlines during the height of the pandemic, the frequency of events has since rebounded to pre-COVID levels. Consequently, these and other mass public shootings have reinforced the need to do more to prevent these tragedies from occurring and to mitigate the harms to individuals and communities if and when they happen. To achieve these goals, it is important to understand the trends associated with the phenomenon of mass public shootings.

The Regional Gun Violence Research Consortium (the Consortium) first published a report in 2018 analyzing 51 years (1966–2016) of mass public shootings data.1 In order to better understand the phenomenon of mass shootings, the report presented information on the location of shootings, weapons used, and demographics of the perpetrators. A follow-up report released in 2021 integrated an additional four years of data (2017–20).2 These reports were based on a comprehensive database of US mass public shootings from researchers Jaclyn Schildkraut and H. Jaymi Elsass.3 As the United States continues to face record rates of gun violence,4 with mass public shootings also increasing in frequency, the need for evidence-based policies is all the more important. A starting point, however, is understanding the different contexts and characteristics of various forms of firearm violence. Mass public shootings, although among the rarest forms of gun violence, require different strategies for prevention and response from other incidents. These shootings often involve considerable planning, which can provide important opportunities for identification, intervention, and de-escalation of the threat before it is carried out, unlike other forms of more spontaneous gun violence.5 Additionally, mass public shootings often are random in nature and occur in large, open public spaces, which present different challenges— including preventative security measures and law enforcement responses—from targeted incidents and those that occur in private locations.

Albany, NY: Rockefeller Institute of Government, 2023. 28p.

By Leslie M. Barnard, Erin Wright-Kelly, Ashley Brooks-Russell; Marian E. Betz

The US has more than 10 times the number of mass shooting events as other developed countries.1 Mass shootings in the US have increased in frequency, with more than half occurring since the year 2000.2 These events have a direct toll on individuals injured or killed, as well as a psychological impact on families, friends, and society.3 Little research has examined the types and distribution of mass shooting events across the US.4 A geographic analysis by type may inform if specific events have disproportionately occurred in particular states or regions of the US. This may generate hypotheses about the contextual (policy, environmental, or sociocultural) factors that may be associated with the distribution of types of mass shooting events and may suggest recommendations for tailored prevention. The purpose of this study was to examine state rates of mass shooting event types and total injuries and deaths in the US.

JAMA Open 2023.

USED BOOK. MAY CONTAIN MARK-UP

By David Cullen

FROM THE COVER: “On April 20, 1999, two boys left an indelible stamp on the American psyche. Their goal was simple: to blow up their school, Oklahoma City-style, and to leave "a lasting impression on the world." Their bombs failed, but the ensuing shooting defined a new era of school violence irrevocably branding every subsequent shooting "another Columbine." When we think of Columbine, we think of the Trench Coat Mafia; we think of Cassie Bernall, the girl we thought professed her faith before she was shot; and we think of the boy pulling himself out of a school windowthe whole world was watching him. Now, in a riveting piece of journalism nearly ten years in the making, comes the story none of us knew. In this revelatory book, Dave Cullen has delivered a profile of teenage killers that goes to the heart of psychopathology. He lays bare the callous brutality of mastermind Eric Harris and the quavering, suicidal Dylan Klebold, who went to the prom three days earlier and obsessed about love in his journal. The result is an astonishing account of two good students with lots of friends, who were secretly stockpiling a basement cache of weapons, recording their raging hatred, and manipulating every adult who got in their way. They left signs everywhere, described by Cullen with a keen investigative eye and psychological acumen. Drawing on hundreds of interviews, thousands of pages of police files, FBI psychologists, and the boys' tapes…..”

NY. Hachette Books. 2009. 423p.

By Sarah Watson

Mass shootings are one example of a focusing event that has particular significance for firearms legislation. Mass shootings shock, disturb and provoke enormous and controversial debate, often causing significant public and media resonance, becoming the subject of intense discussion politically. At times providing an impetus for legislative amendments, often in distinct ways that routine gun violence does not. If certain events highlight the need for reform, policy change becomes more likely. Cases with the lowest number of victims are likely to generate the least amount of attention and are most likely to be missed in data collection, rendering them the least noteworthy, least important in terms of lethality and social and political consequence. Various problems come to the attention of people in and around government, necessitating an understanding of why such problems occupy officials’ attention and appear to be more ‘deserving’ of attention.

Criminology & Criminal JusticeOnlineFirst, July 19, 2022

By The Joint Federal/Provincial Commission into the April 2020 Nova Scotia Mass Casualty

  Volume 4 addresses several issues set out in the Commission’s mandate, including the direction to inquire into “the steps taken to inform, support and engage victims, families, and affected citizens.” We also develop additional main findings as well as lessons learned and recommendations pertaining to the direction to examine these issues set out in the Orders in Council: (ii) access to firearms (v) communications with the public during and after the event, including the appropriate use of the public alerting system established under the Alert Ready program (ix) policies with respect to the disposal of police vehicles and any associated equipment, kit, and clothing (xi) information and support provided to the families of victims, affected citizens, police personnel, and the community. Volume 4, Community, focuses on the role of communities and their members in responding to critical incidents and in contributing to community safety and wellbeing. We open our report by recognizing the extensive harm resulting from the April 2020 mass casualty, one that centres on the lives taken, the survivors, and their families; and ripples out to affected communities, emergency responders, and outward through a circular pattern of impact. One of our main findings in Volume 2 is that community members played a central role as first responders during the mass casualty. We also find the harms caused by the perpetrator’s violence did not begin on April 18. in Volume 3 we bring together these two findings and underscore the ways in which community members are also first preventers, in the sense of fostering the safety of women and others affected by gender-based violence. This volume elaborates this understanding of the dual community role. Communities and their members are both affected by critical incident responses and by systems for ensuring everyday safety, and they have an active role in responding to  incidents and in contributing to community safety and well-being on an ongoing basis. Perhaps more fundamentally, the structure of our Report recognizes that we need to rebalance the relationship between communities and police in ensuring public safety. To put it simply, communities come first.     

 Halifax, NS: Joint Federal/Provincial Commission into the April 2020 Nova Scotia Mass Casualty, 2023. 734p.

By Alathari, Lina; Drysdale, Diana; Driscoll, Steven; Carlock, Arna L.; Cutler, Meagan; Besser, Jason

From the Executive Summary: "As law enforcement agencies, workplaces, and other community organizations implement behavioral threat assessment programs, the approach should be guided by the research findings contained in this report. When conducted properly, a behavioral threat assessment will involve promoting bystander reporting to identify warning signs of potential violence, systematically gathering information about the circumstances and behaviors of concern, assessing the possibility of violence as an outcome, and implementing preventive management strategies to make positive and safe outcomes more likely. The 173 attacks contained in this report impacted a variety of locations, 'including businesses/workplaces, schools, houses of worship, military bases, nonprofit service providers, residential complexes, public transportation, and open spaces'. In many cases, the attacker had a known affiliation with the site of the attack. The analysis is intended to provide critical information to a cross-sector of community organizations that have a role in preventing these types of tragedies."

Washington. US. United States. Secret Service. National Threat Assessment Center. 2023. 70p.

By United States. Department of Justice. Office of Community Oriented Policing Services

From the Preface: "Nine mayors whose cities have experienced mass shootings in recent years attended the Winter Meeting of The U.S. Conference of Mayors, held in January 2020 in Washington, D.C. Their presence in that meeting offered an opportunity for them, as a group, to share both their experiences and their personal feelings about what was required of them as mayors. Their goal in that session was to alert their audience to the need to prepare for a mass shooting; as many veterans of these tragedies agree, the question must not be if your city will experience a mass shooting, but 'when'. This report captures many of the key points offered that day by those mayors and later by key city staff involved in the response to one of the deadliest mass shooting incidents in our history. It is intended to provide guidance on the preparation that should take place in anticipation of an incident occurring, the critical importance of communication during the response to an incident, and the wide range of tasks that must be anticipated in the incident's wake. Sadly, while it is offered to all mayors in the hope that they will never have to call upon the preparation that is urged, it is likely that in the months and years ahead we all will be reaching out to some who will."

U.S. Conference of Mayors. Washington D.C. January 2020. 20p.

By Rosanna Smart, Terry L. Schell

There is no standard definition of what constitutes a mass shooting, and different data sources—such as media outlets, academic researchers, and law enforcement agencies—frequently use different definitions when discussing and analyzing mass shootings. For instance, when various organizations measure and report on mass shootings, the criteria they use in counting such events might differ by the minimum threshold for the number of victims, whether the victim count includes those who were not fatally injured, where the shooting occurred, whether the shooting occurred in connection to another crime, and the relationship between the shooter and the victims. These inconsistencies lead to different assessments of how frequently mass shootings occur and whether they are more common now than they were a decade or two ago. Data show that, regardless of how one defines mass shootings, perpetrators are likely to be men. But several other characteristics that are statistically predictive of perpetration are still uncommon among offenders on an absolute level. The rare nature of mass shootings creates challenges for accurately identifying salient predictors of risk and limits statistical power for detecting which policies may be effective in reducing mass shooting incidence or lethality. Implementing broader violence prevention strategies rather than focusing specifically on the most-extreme forms of such violence may be effective at reducing the occurrence and lethality of mass shootings.

Santa Monica, CA: RAND, 2021. 32p.

By Jillian Peterson

This is the Final Report on the goals and achievements of a project that created a database on patterns in the lives of mass shooters to assist in informing policymakers and practitioners in developing and implementing prevention and intervention strategies for mass shootings. Informed by existing datasets, the research literature, and frequently asked questions about mass shooters, this project compiled a list of variables to be coded and developed a codebook. The codebook was piloted on a small random sample of test cases and refined. Open-source lists of mass shootings were developed from major news outlets. Among the variable categories are leakage of the killers’ plans, crisis signs, victim characteristics, firearms possession and use, role of psychosis in the shooting, behaviors related to violence, mental health, family characteristics, and social and interest groups. The report also contains findings from the database on mass shooting trends; shooter demographics; trauma, suicidality, and crisis; mental health; and warning signs

St. Paul, MN: Hamline University, 2021. 38p.

Jillian J. Turanovic, Travis C. Pratt, Kristen Neville, and Antonia La Tosa

“…mass shootings encompass all incidents with four or more gunshot fatalities, not including the shooter, within 24 hours. Existing publicly available databases on deadly mass shootings often focus only on those that occur in public spaces (and that are neither family- nor felony-related). We include public mass shooting incidents, but we also assess all the other deadly mass shootings that occurred over this period (e.g., those that happened in private spaces among family members, and those that occurred in public spaces but were part of another criminal event). Our effort to be more inclusive allows for the examination of not only the characteristics of deadly mass shootings that occur in public, but to also examine how similar (or not) these incidents are to deadly mass shootings that take place in other contexts. In all, our database contains information on 720 incidents that occurred in both public and private spaces between 1980 and 2018. Consistent with prior research, we used open-source data (e.g., media reports, official police and court records) to code each deadly mass shooting incident according to a host of characteristics.

Tallahassee: College of Criminology and Criminal Justice, Florida State University , 2022. 53p.

By Nils Duquet

Every year in Europe around 1,150 people are shot dead with firearms. These deaths occur in various contexts, including the relational and criminal spheres. The little available research into the weapons used during these fatal crimes suggests that different types of firearms are used in different contexts.1 In this report we focus on the weapons that were used for one specific form of deadly firearms incidents, namely “public mass shootings”. These are shooting incidents in the (semi-)public space in which the perpetrator(s) use one or more firearms and during which several people are killed and injured. Under this umbrella term we find numerous types of shooting incidents, ranging from school shootings in which a frustrated pupil murders a number of classmates and/or teachers to terrorist attacks in which groups of perpetrators attempt to spread terror in coordinated attacks. A look at recent mass shootings shows that significant differences can be observed in terms of, for example, the locations where these shootings were carried out, the number of perpetrators, the motives of the perpetrator(s), the selection of the victims and the firearms used. This report analyses one specific aspect of such shooting incidents, namely the firearms that were used.

Brussels: Flemish Peace Institute, 2016. 45p.