Mass Shooters

The Violence Project Mass Shooter Database

Funded by the National Institute of Justice (award number 2018-75-CX-0023), and recipient of international media attention, the public mass shooter database project includes four phases:

1. Creation of a comprehensive database of over 150 mass public shootings from 1966 to 2018 coded through over 70 individual-level psycho-social life history variables, including mental health history, trauma, interest in past shootings, and situational triggers.

2. 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 guns in households.

3. 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.

4. Dissemination of findings, creation of a 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 Public Mass Shooting?

America 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. 112 of those deaths were attributable to public mass 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 public 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. They are rare but focusing events.

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 traumatic 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 by the shooter, the number of rounds fired, proximity to the nearest Level 1 Trauma Center, if/how many bullets hit vital organs (which can be a matter of millimeters) etc. A broader definition with fewer deaths or non-fatal injuries or the death of the shooter included would certainly yield much higher numbers. For example, see other databases from our friends at CHDS K-12 School Shooting Database, Crime Prevention Research Center, Everytown for Gun Safety, The Gun Violence Archive, Mother Jones, The New York Times, Security Baron, Stanford University, Supplementary Homicide Reports (FBI), USA Today, Vox, The Wall Street Journal, and The Washington Post.

Other databases focus almost exclusively on spatial and temporal data—the what, when, and where of public 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.

The full database will be publicly available by January 2020.


The database is comprised of publicly available information. There are inherent biases and obvious limitations with this approach. There are missing data and data gathered for purposes different from our own. We know more about more recent cases, which may simply reflect increased awareness of mass shootings or more resources being dedicated to covering and countering them. Still, we have taken every step possible to find and verify sources and to rigorously fact-check the data. We have consulted existing databases, read media reports and social media posts, books and peer-reviewed articles, manifestos and suicide notes, trial transcripts and medical records. The following variables are all coded in the mass shooter database:


  • Name
  • Age
  • Gender
  • Race
  • Education
  • Citizenship / immigration status
  • Sexual identity
  • Marital or relationship status
  • Children
  • Employment status
  • Job type
  • Religious affiliation
  • Military status
  • Gang affiliation
  • Terror or hate group affiliation

Shooting location

  • Region of country (Northeast, Midwest, South, West)
  • City, State
  • Site (e.g., School, House of Worship)
  • Site specifics
  • Bifurcated site
  • Did shooter frequent that space?
  • Nearest hospital / Level 1 trauma center

Life history

  • Health issues
  • Mental illness diagnosis
  • Prior counseling
  • Prior psychiatric hospitalization
  • Prior psychiatric medication
  • Adverse Childhood Experiences (ACEs)
  • Adult trauma
  • Single-parent household
  • Parental suicide
  • Used insanity defense

Crisis point

  • Recent change in relationship status
  • Recent change in employment status
  • Bullied at school
  • Suicidal

Warning signs

  • Criminal record
  • Signs of a crisis
  • Leakage before shooting
  • Degree of planning
  • Interest in past violence
  • Interest in firearms
  • History of violence
  • Bully at school


  • Relationship with other mass shootings
  • Something left behind (tokens, manifestos, etc.)
  • Social media use
  • Performance (will to representation)


  • Specific grievance
  • Domestic spillage
  • Misogyny
  • Hate / racism

Event characteristics

  • Date
  • Start and end time
  • Time of day
  • Security
  • Armed person on scene
  • Resolution (escaped, suicide, surrendered, apprehended, killed)
  • Kidnapping or hostages


  • N killed
  • N injured
  • Known or unknown to shooter


  • N brought to scene
  • Firearm types:
    • N handguns
    • N shotguns
    • N rifles
    • N assault rifles
  • Other weapons
  • How weapons were obtained

Community factors

  • Urban/Suburban/Rural
  • Population
  • Median age
  • Racial breakdown
  • % foreign born
  • % female head of household
  • % rental units
  • % unemployed
  • % high school graduate
  • % college graduate
  • % without medical insurance
  • N mental health providers in zip code
  • N gun stores in zip code
  • Size of police department
  • Homicide rate

Why mass shootings occur, and how we can all stop them

Our research has led us to a new, hopeful, framework to understand public mass shootings and how to prevent them. Mass shooters typically have four things in common:

  1. Early childhood trauma and/or exposure to violence at a young age;
  2. An identifiable grievance and/or crisis point;
  3. Validation for their belief system, have studied past shootings to find inspiration;
  4. The means to carry out an attack (access to people, places, and firearms).

This new framework acknowledges the complexity of this vexing social problem, but gives all of us things we can do to prevent the next tragedy. Each one of the four themes represents an inflection point—an opportunity for intervention.

Read more here:

Peterson, J. & Densley, J. (2019, Aug. 4). We have studied every mass shooting since 1966. Here’s what we’ve learned about the shooters. Los Angeles Times.

Gun Violence in America

The Violence Project has produced a 50 page comprehensive report on gun violence in America. This free report examines the history of guns in the United States, the prevalence of gun violence in our communities, trends in mass shootings, and the rise of “performance violence” in an age of social media. This report also analyses what works in gun violence prevention and intervention. Download here.

Densley, J. & Peterson, J. (2017). Gun violence in America. St. Paul, MN: The Violence Project LLC. doi: 10.13140/RG.2.2.29196.00649