Disclaimer: This project was supported by Award No. 2018-75-CX-0023, awarded by the National Institute of Justice, Office of Justice Programs, U.S. Department of Justice. The opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect those of the Department of Justice.
The mass shooter database project included four phases:
- Creation of a comprehensive database of mass public shooters from 1966 to today coded on over 100 life history variables, including mental health history, trauma, interest in past shootings, and situational triggers.
- Examination of community-level socio-ecological factors of where mass public shootings take place, including, but not limited to, crime rates, measures of social inequality, community mobility, availability of mental health resources, and prevalence of gun stores.
- In-depth life history interviews with living mass shooters who are currently incarcerated and follow-up interviews with key stakeholders (e.g., family members, first responders, survivors, experts) in the communities where shootings took place.
- Dissemination of findings, creation of this public website, and implications for evidence-based prevention strategies.
The work is guided by rigorous ethical protocols from the Institutional Review Board at Hamline University and a steadfast commitment to no notoriety for mass shooters.
What is a Mass Shooting?
There are several gun violence problems in the United States, each demanding our attention and action, including:
- Suicide by firearm;
- Everyday gun violence (i.e., interpersonal conflicts and shootings related to gangs/groups or underlying crime);
- Domestic shootings;
- Unintentional shootings;
- Police-involved shootings;
- Mass public shootings.
Some problems are more prevalent and pressing than others, but each has its own victim and offender profile, underlying mechanisms, and potential solutions.
There is no universally accepted definition of a mass shooting. For a long time, mass shootings were studied as a variation on "mass killing", the generic term for multiple homicides that vary along what researchers agree are important dimensions: time, place, and method. For instance, someone who kills their victims in separate events is different from someone who kills them all at once. A person who kills in public is different from a person who kills in private (especially when private victims tend to be family members or intimate partners); and different still from a contract killer, bank robber, or gang member who kills in the commission of another crime. And an arsonist or bomber is different from a shooter.
Some sources define a mass shooting as any incident in which three or more victims were shot and killed, not including the perpetrator, while others use a threshold of four or more killed. Other sources define a mass shooting as any incident in which four or more people were shot or injured, which greatly increases the incident count. Some sources include domestic violence, gang conflict, drug trade disputes, or robberies in their numbers. At The Violence Project, we focus on mass public shootings, defined by the Congressional Research Service as follows:
“a multiple homicide incident in which four or more victims are murdered with firearms—not including the offender(s)—within one event, and at least some of the murders occurred in a public location or locations in close geographical proximity (e.g., a workplace, school, restaurant, or other public settings), and the murders are not attributable to any other underlying criminal activity or commonplace circumstance (armed robbery, criminal competition, insurance fraud, argument, or romantic triangle).”
We acknowledge the limits of this definition. Every mass casualty event is a tragedy and many factors influence whether a threshold of four or more people killed is reached, including the accuracy of the shooter, the type and caliber of the firearm used, the number of rounds fired, the actions of first responders, proximity to the nearest hospital, and if/how many bullets hit vital organs. Any cut point is arbitrary, but this remains a widely agreed-upon standard. Further, the number of deaths is the strongest predictor of media coverage, which we use to help build our database.
By focusing only on mass public shootings, we exclude domestic homicides (although if 50% or more of victims are non-relatives killed in public then we include them). We also exclude mass shootings attributable to gangs or underlying criminal activity (e.g., a robbery or drug deal gone bad). A broader definition with a threshold of fewer deaths, non-fatal shootings, or any means/motive would certainly yield more cases. For examples, see:
| Source | Victim Threshold | Inclusion/Exclusion Criteria | Time/Space | Years |
| ALERRT Active Attack Data | N/A | “An individual, or individuals, actively killing or attempting to kill multiple unrelated people in a public space.” | Public | 2000–2021 |
| AP/USA TODAY/Northeastern University | 4+ killed, excluding unborn children & offender(s) | none | Any, within a 24-hour period | 2006–present |
| Everytown for Gun Safety | 4+ killed, excluding offender(s) | none | Any | 2009–present |
| FBI | N/A | “An active shooter is an individual actively engaged in killing or attempting to kill people in a populated area.” | “Populated area” | 2000–2021 |
| Greene-Colozzi and Silva (2022) | 4+ killed or the offender must have “demonstrate behavioral evidence suggesting mass victim intent” | Excludes familicide or shootings related to other criminal activity. Victims must be chosen at random or for their symbolic value | "at one or more public or populated locations" within a 24-hour period | 1966–2019 |
| Gun Violence Archive | 4+ shot, including offender(s) | none | single event, at the same general time and location | 2014–present |
| Mother Jones | 3+ killed, excluding offender(s) | Excludes domestic or “conventionally motivated crimes” such as gang violence and armed robbery | Public, within one event | 1982–present |
| Schildkraut and Elsass (2022) | “Multiple victims (both injuries and fatalities)” | “must not correlate with gang violence or targeted militant or terroristic activity” Victims must be chosen at random or for their symbolic value | “at one or more public or populated locations” within a single 24-hour period | 1966–2020 |
| Stanford Mass Shootings in America | 3+ shot, excluding the offender(s) | Excludes “identifiably gang, drug or organized crime related shootings” | Public, within a 24-hour period | 1982–2017 |
| The Violence Project | 4+ killed, excluding the offender(s) | Excludes domestic/felony-related incidents | Public, within one event | 1966–present |
| The Washington Post | 4+ killed, excluding the offender(s) | Excludes domestic/felony-related incidents | Public, within one event | 1966–2021 |
Our database was primarily built to explore the life histories of mass shooters, not simply track mass shootings. Our main unit of analysis is the shooter, not the shooting. This means that in addition to the what, when, and where of mass public shootings, we study the why and how, plus who the victims of these crimes were, hopefully moving us closer to a world without mass shootings.
Building The Database
To build the database, we used the following:
Primary Sources:
- Written journals / "manifestos" / suicide notes etc.
- Social media and blog posts
- Audio and video recordings
- Interview transcripts
- Personal correspondence with perpetrators
Secondary Sources (all publicly available):
- Media (television, newspapers, magazines)
- Documentary films
- Biographies
- Monographs
- Peer-reviewed journal articles
- Court transcripts
- Police reports
- Medical records
- School records
- Autopsy reports
Community Variables:
- U.S. Census Bureau
- FBI Uniform Crime Reports
- Google Maps