Funded by the National Institute of Justice (award number 2018-75-CX-0023), and recipient of global media attention, the public mass shooter database project includes four phases:
- Creation of a comprehensive database of all mass public shootings from 1966 to 2020 coded on nearly 200 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?
The United States has not one gun violence problem, but several. Everyday gun violence claims or changes hundreds of lives each week, disproportionately young Black and Latino men. In 2017 alone, the Centers for Disease Control reported 14,542 homicides by discharge of firearms. About 106 of those deaths were attributable to mass public shootings, according to our data—the highest of any year recorded because of the Las Vegas shooting that claimed an unprecedented 58 lives. The fact that mass shootings account for fewer than 1% of all firearm homicides does not diminish their extraordinary tragedy—mass shootings cause damage far beyond that which is measured in lives lost. It merely highlights that mass shootings are focusing events.
A mass shooting is a variant on "mass murder," but the more generic term lumps together cases that vary along what researchers agree are important dimensions: time, place, and method. 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 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.
There is no universally accepted definition of a mass shooting. We follow the Congressional Research Service definition, which is quite conservative:
“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 weapon used, the number of rounds fired, proximity to the nearest hospital, and if/how many bullets hit vital organs. Any cut point is arbitrary, but this is the standard agreed upon by researchers and the FBI. Further, the number of deaths is the strongest predictor of media coverage, which is necessary to accurately track mass shootings.
By focusing only on public events, we exclude domestic mass shootings (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 or motive would certainly yield more cases. For examples, see:
| Source | Dates | Casualties | Location | Exclusions |
| Violence Project | 1966–
present |
4+ killed
(excludes shooter) |
Public | When victims are exclusively family or friends;
shootings related to gangs or underlying crime |
| Gun Violence | 2014–
present |
4+ shot | Any | None |
| AP/USA Today/ | 2006–
present |
4+ killed
(excludes shooter) |
Any | None |
| Everytown for Gun Safety | 2009–
2018 |
4+ killed | Any | None |
| Stanford Mass Shootings of America | 1966–
2016 |
3+ shot
(excludes shooter) |
Any | Shootings related to gangs, drugs, or organized crime |
| Washington Post | 1966–
present |
4+ killed
(excludes shooter) |
Public | When victims are exclusively family or friends;
shootings related to gangs or underlying crime |
| Mother Jones | 1982–
present |
3+ killed
(4+ killed ≤ 2013) |
Public | “Conventionally motivated crimes” such as gang violence and armed robbery |
Other databases focus almost exclusively on spatial and temporal data—the what, when, and where of mass shootings. Our database is focused on that and everything else, moving us closer to the why and how and, in turn, finding pathways to prevention.
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
- Law Enforcement records
- Medical records
- School records
- Autopsy reports
Census Variables:
- U.S. Census Bureau
- FBI Uniform Crime Reports
- Google Maps