Mass murder

Mass murder is the violent crime of killing a number of people, typically simultaneously or over a relatively short period of time and in close geographic proximity. A mass murder typically occurs in a single location where one or more persons kill several others. Data suggests that approximately 30% of mass murderers die in the act.

In the United States, Congress defined mass murders as the killing of three or more persons during an event with no "cooling-off period" between the homicides. The Investigative Assistance for Violent Crimes Act of 2012, passed in the aftermath of the Sandy Hook Elementary School shooting, clarified the statutory authority for federal law enforcement agencies, including those in the Departments of Justice and Homeland Security, to assist state law enforcement agencies, and mandated across federal agencies a definition of "mass killing" as three or more killings during an incident. A mass murder may be further classified as a mass shooting or a mass stabbing. Mass murderers differ from spree killers, who kill at two or more locations with almost no time break between murders and are not defined by the number of victims, and serial killers, who kill people over long periods of time.

Many terrorist groups in recent times have used the tactic of killing many victims to fulfil their political aims. Such incidents have included:

Certain cults, especially religious cults, have committed a number of mass killings and mass murder–suicides.

Mass murderers may be categorized into killers of family, of coworkers, of students, and of random strangers. Their motives vary. One motivation for mass murder is revenge, but other motivations are possible, including the need for attention or fame.

Several autocratic regimes have been known to have ordered mass murders including genocide and politicide. Acting on the orders of Soviet leader Joseph Stalin, Vasili Blokhin's war crime of killing of 7,000 Polish prisoners of war, shot over 28 days, was one of the most organized and protracted mass murders by a single individual on record.

Analysis of the Columbine High School massacre and other incidents where law enforcement officers waited for backup has resulted in changed recommendations regarding what victims, bystanders, and law enforcement officers should do. In the Columbine shooting, the perpetrators, Eric Harris and Dylan Klebold, were able to murder 14 people, then commit suicide before the first SWAT team even entered the school. Average response time by law enforcement to a mass shooting is typically much longer than the time the shooter is engaged in killing. While immediate action may be extremely dangerous, it could save lives which would be lost if victims and bystanders involved in the situation remain passive, or law enforcement response is delayed until overwhelming force can be deployed. It is recommended that victims and bystanders involved in the incident take active steps to flee, hide, or fight the shooter and that law enforcement officers present or first arriving at the scene attempt immediately to engage the shooter. In many instances, immediate action by victims, bystanders, or law enforcement officers has saved lives. However, law enforcement programs and actions have so far been unable to reduce the total number of incidents. In 2020, a record number of 600 mass shootings occurred.

Commentators have pointed out that there are a wide variety of ways that homicides with more than several victims might be classified. Such incidents can be, and have been even in recent decades, classified many different ways including (per Joel Best of Reason magazine) "as a mass shooting; as a school shooting; as mass murder; as workplace violence ...; as a crime involving an assault rifle; as a case of a mentally ill person committing acts of violence; and so on."