Mass killing database: Revealing trends, details and anguish of every US event since 2006

A partnership with The Associated Press and Northeastern University

The horror and tragedy of mass shootings in American schools, churches and other public places capture the nation's attention. But these are only part of the larger violence of mass killings – deaths by guns, knives, fires, vehicles and other weapons in public and in private – that plague the U.S., research shows.

Over the past decade, USA TODAY, along with Northeastern University and The Associated Press, has been tracking all mass killings in the United States. When it comes to gun violence, our database is narrower than some tracking sites, such as the Gun Violence Archive, that include shootings that injure large numbers of people but kill no one. However, our database is broader in other ways. It includes every mass killing since 2006 from all weapons in which four or more people, excluding the offender, were killed within a 24-hour time frame. The database also includes dozens of variables on each incident, offender, victim, and weapon.

2023 saw the second most mass killings in a single year since tracking began. It also had the most mass shootings (39) and the most public mass shootings (10). The number of victims in 2023 was the third highest total, only surpassed in 2017 and 2019.

James Alan Fox, a professor of criminology, law and public policy at Northeastern University in Boston, oversees the database and continues to add information drawn from media reports, FBI data, arrest records, medical examiners' reports, prison records and other court documents. Cases are followed from the date of occurrence through the sentencing of perpetrators charged in each crime.

According to Dr. Fox, there is some disagreement as to whether the rate of mass killing constitutes an epidemic, as some observers have characterized it, but there is no doubt that it remains a significant problem in the U.S., sparking fear and anxiety across all corners of the nation.

This is what we have learned:

Public mass shootings are only part of the story

Cases in which someone shoots strangers in a public place usually get the most attention. But fatal public shootings are a small fraction of all mass killings. There has been a spike in these types of killings over the past few years, but the rate of occurrence has remained relatively flat since the mid-2000s, especially considering the growth in the U.S. population over that time frame.

“A guy who kills his wife and children and sometimes then kills himself is the most common type of mass killing,” Fox said. Mass killings take place far more often in private homes than in schools, supermarkets or churches.

Victims of mass killings are more likely to have been killed by someone they know

Mass shootings in which family members or intimate partners are targeted are twice as common as fatal public mass shootings in which strangers are killed. In our database, the public category excludes killings related to drug deals, gang disputes, robberies and other criminal activity, which are instead classified as being related to a felony or suspected felony, depending on the circumstances.

On average, our data shows that five people die in a mass killing of any kind, compared with seven in a fatal public mass shooting. Though these public killings result in more per-incident deaths, other types of mass killings happen more frequently and result in more victim fatalities and injuries overall.

Other data sources corroborate the outsize role of family mass killings. According to a study by the gun control group Everytown for Gun Safety, nearly 3 in 4 children and teens killed in mass shootings over the past decade died in an incident connected to domestic violence.

Guns surpassed car accidents as the leading cause of death for children and teenagers in 2020. The school security industry has grown into a $3.1-billion-a-year market, but our database shows that when it comes to mass killings, most children die at home.

Most mass killings are committed using handguns

The majority of mass killings are commited with firearms. These events usually happen about two dozen times a year. Analysis shows non-gun-related deaths make up just under 20% of mass killings.

Semiautomatic handguns are far more common in mass killings than guns that are typically characterized as assault weapons, such as the AR-15. According to Fox, handguns are easily concealable and some can be equipped with large-capacity magazines. In this database, the long guns category includes any gun larger than a handgun, including rifles and shotguns.

Far more people kill themselves with a firearm each year than are killed with one, according to statistics from the Centers for Disease Control and Prevention analyzed by Everytown for Gun Safety. Nearly two-thirds of all gun deaths in the U.S. are suicides. More than half of suicides involve guns. Since 2006, firearm suicide rates have been steadily increasing.

Men who own handguns are almost eight times more likely to die of gun suicides than men who don’t, according to a California study published in 2021 by the New England Journal of Medicine that examined more than 25 million gun owners. Women were 35 times more likely to commit suicide if they own a handgun.

Mass killers often take their own live after shooting their victims. About 30% of mass shooters commit suicide, and that figure rises to just over half in mass killings involving family members or intimate partners.

Most mass killers are men

According to one study by James Knoll and Ronald Pies in the medical trade publication Psychiatric Times, mass killers are typically angry, aggrieved, emotionally unstable people who seek retribution or revenge for perceived mistreatment, rejection or humiliation. Offenders are often socially isolated and have limited social support.“The ones who are more apt to commit mass murder are the ones who are paranoid, and who are suspicious and mistrusting and think that everyone is against them,” said Michael Stone, a forensic psychiatrist at Columbia University who maintains a database of 350 mass killers going back more than a century. “It’s somebody who has fewer capacities at his disposal for adjusting to the crisis of losing the job or the girlfriend and will feel that life is over.”

Four in 10 men report that they own a gun. Gun owners are also predominantly white. According to Stone, men tend to be more aggressive and are more prone to violence. Offenders who commit public mass killings tend to be younger. At the same time, most family homicides involve middle-aged men.

“As far as the offender age goes, for the family killings, you don't have 21-year-olds who have a wife and three kids,” Fox said.

With a few exceptions, victims and offenders in mass killings tend to reflect the population. White Americans make up the largest overall percentages of mass killing victims and offenders—approximately 50% of victims and 40% of offenders. White perpetrators commit about half of all family mass killings and about 55% of public mass killings.

In felony mass killings – we define these as being associated with known or suspected criminal activity such as robbery, illicit drug trade or gang conflict – Black perpetrators commit just over 50% of these crimes while Hispanic offenders commit about 20% of them, both more than their population shares.

Mass killings aren't confined to big cities

Mass killings take place across the country in cities and towns of all sizes. Homicides with fewer than four victims are more common in larger cities, but mass killings with higher death tolls often take place in smaller towns or rural settings.

Why do people commit mass killings?

Mass killers typically target certain people for a variety of specific reasons, and can be precipitated by such traumatic events as the end of relationship or the loss of a job. Mass killings are rarely indiscriminate. Also, mass killings tend not to be a spontaneous act of violence as perpetrators usually plan their assaults for days, weeks or months.

According to a studyby Melanie Taylor in the International Journal of Offender Therapy and Comparative Criminology, a failed or estrangedrelationship is the most common reasons for committing a mass killing. Perpetrators are often dealing with relationship and financial struggles for an extended time, and a particularly traumatic event can push them to kill.

The motive for a mass killing is often revenge. Most commonly, the killer seeks to get even with people they know in retaliation for real or imagined slights. “Sometimes it's marital discord where a man kills the wife out of anger, and the children as well by seeing them as being associated with the wife,” Fox said.

There are also mass killings in which disgruntled employees kill their managers and other co-workers. But these attacks happen rarely – one or two a year, according to Fox. In the case of losing a job, a mass killing usually results in the slaying of family members.

Other common motivations include gaining control, carrying out a robbery or sending a message by committing a killing, according to Fox. Perpetrators sometimes target a certain group of people, such as immigrants or people of a single race, but target individuals within those groups at random. Some mass killings happen for reasons that cannot be immediately or easily identified, particularly if the assailant commits suicide or is killed at the scene.

“In the aftermath of a mass killing, all those missed warning signs become crystal-clear when hindsight is 20/20,” Fox said. “It is really not possible to predict in a reliable way those who will commit a mass killing.”

Profiles of mass killers reveal common elements shared by many people in the general population. Countless Americans may be angry, frustrated, reclusive, quick to blame others for their shortcomings, willing to post hateful messages on social media sites, or make threats – but relatively few will kill, much less commit mass killings.“One misconception is that they are easy to spot, that a mass killer looks like mass killers are supposed to look,” Fox said. “But that’s not the case. They oftentimes are extraordinarily ordinary, you wouldn’t suspect.”

Mass killers usually don't have criminal records. Researchers at Columbia University analyzed 1,315 mass murders of all types worldwide between 1900 and 2019 and found that 20% of mass killers had histories of being subject to a restraining order, arrest or incarceration.A study by Emma E. Fridel in the Journal of Interpersonal Violence that examined all mass murders between 2006 and 2016 found that 17% of offenders had a history of domestic violence. For killings in public places, the number was 7%. It is quite rare for a heated dispute to escalate into a mass killing.

Several studies confirm that most shooters who kill four or more victims are typically not psychotic or hallucinatory and haven't been treated for mental health problems. An analysis of Columbia University's mass murder database shows that 11% of all mass murderers had serious mental illness.

“There's no mental illness whose symptom is hurting someone else, let alone shooting somebody else,” said Jonathan Metzl, a professor of psychiatry at Vanderbilt University.“Those illnesses are marked by low energy and low mood and poor cognitive planning sometimes, because they interfere with the way you think,” Metzl said. “So planning something as complicated as a mass shooting is not going to be the easiest thing.”

Explore the database

About the data

The USA TODAY/AP/Northeastern University mass killing database contains information on incidents, offenders, victims and weapons for all multiple homicides with four or more victims killed in the United States from 2006 to the present.

Definitions

A mass killing is defined as the intentional killing of four or more victims – excluding the deaths of unborn children and the offender(s) – by any means within a 24-hour period.

This definition includes cases involving all weapons (shooting, blunt force, stabbing, explosives), types (public, felony-related, and familicides), motivations (domestic dispute, profit, revenge, terrorism, hate), victim-offender relationships (stranger, family, acquaintance, co-worker), and number of locations. The time frame of 24 hours was chosen to eliminate conflation with spree killers who kill multiple victims over several days in different locations and to satisfy the traditional requirement of occurring in a “single incident,” even if that incident involves an offender targeting multiple locations in an extended assault but within a relatively short time span. However, offenders who kill four or more victims during any 24-hour period of time as part of a multi-day spree are included, as are all their victims within seven days of the mass killing. Negligent homicides related to driving under the influence or accidental fires are excluded because of the lack of intent. Finally, only incidents occurring within the 50 U.S. states and the District of Columbia are included in the database.

Consistent with the traditional definition, fatal mass shootings are mass killings (four or more victim fatalities) in which most or all the victims are killed by gunfire. This differs from an alternative definition used by the Gun Violence Archive that includes incidents in which at least four victims are shot regardless of whether the injury is fatal. Less than 5% of the mass shootings listed in the Gun Violence Archive are defined as mass killings in our database. Our definition of a fatal mass shooting also differs from an active shooter event which, as characterized by the FBI, involves an individual actively engaged in killing or attempting to kill people in a populated area. Less than 25% of active shooter events result in four or more victim fatalities, constituting a mass killing.

Methods

Researchers at USA TODAY first identified potential incidents using the FBI’s Supplementary Homicide Reports (SHR). Homicide incidents in the SHR were flagged as potential mass killing cases if four or more victims were reported on the same record, and the type of homicide was coded as “murder or non-negligent manslaughter.” Cases were subsequently verified utilizing media accounts, court documents, academic journal articles, books and local law enforcement records obtained through Freedom of Information Act (FOIA) requests. Each data point was corroborated by multiple sources, which were compiled into a single document to assess the quality of information. When sources were contradictory, official law enforcement or court records were used, when available, followed by the most recent media or academic source. Case information was subsequently compared with other available mass killing or mass shooting databases to ensure validity. Incidents listed in the SHR that could not be independently verified were excluded from the database.

In 2016, primary data collection and verification efforts shifted from USA TODAY to Northeastern University. Northeastern researchers conducted extensive searches for incidents not reported in the SHR during the time period, utilizing internet search engines including Lexis-Nexis, Google News, and Newspapers.com. Search terms included: [number] dead, [number] killed, [number] slain, [number] murdered, [number] homicide, mass murder, mass shooting, massacre, rampage, family killing, familicide and arson murder. Offender, victim and location names were also directly searched when available. Northeastern University researchers also independently verified data collected by USA TODAY staff and filled in missing information, sometimes involving updated reports on older cases.

In December 2018, a Memo of Understanding (MOU) was signed by The Associated Press, USA TODAY and Northeastern University to formalize a joint initiative to maintain and expand the mass killing database previously housed at USA TODAY. The Associated Press hosts the database and maintains the data entry tool, USA TODAY has developed and maintains the public website for and visualizations of the database, and Northeastern University manages data collection and updates.

The full database currently consists of four linked data tables with a total of 59 data fields (not counting indicators for the availability of offender/victim identity) –18 fields for each incident, 20 fields for each offender, 13 fields for each victim killed and eight fields for each weapon used. Most variables, with the notable exception of victim names, are available for public download. The remaining data is reserved for individuals affiliated with The Associated Press, USA TODAY/Gannett, and Northeastern University’s School of Criminology and Criminal Justice, and others by permission of all three organizations. Moving forward, additional variables may be added to the full database as well as the public subset. While USA TODAY respects diversity of gender, this database instead uses sex as a datapoint as is common in crime statistics.