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Note: Varies by jurisdiction
Note: Varies by jurisdiction
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Mass murder (in military contexts, sometimes interchangeable with "mass destruction") is the act of murdering a large number of people, typically at the same time or over a relatively short period of time.
According to the FBI, for individuals, mass murder is defined as the person murdering four or more persons during a particular event with no cooling-off period between the murders. A mass murder typically occurs in a single location in which a number of victims are killed by an individual or more. With exceptions, many acts of mass murder end with the death of the perpetrator(s), whether by direct suicide or being killed by law enforcement.
A mass murder differs from a spree killing, in that it may be committed by individuals or organizations, whereas a spree killing is committed by one or two individuals. In terms of individuals, mass murderers are different 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 may kill numerous people over long periods of time. Mass murder is also not synonymous with genocide because genocide requires distinct elements.
Mass murder may also be defined as the intentional and indiscriminate murder of a large number of people by government agents. Examples are the shooting of unarmed protestors, the carpet bombing of cities, the lobbing of grenades into prison cells, and the random execution of civilians. The largest mass killings in history have been governmental attempts to exterminate entire groups or communities of people, often on the basis of ethnicity or religion. Some of these mass murders have been found to be genocides and others to be crimes against humanity, but often such crimes have led to few or no convictions of any type.
Mass murder by a state 
The concept of state-sponsored mass murder covers a range of potential killings. It is defined as the intentional and indiscriminate murder of a large number of people by government agents. Examples are shooting of unarmed protestors, carpet bombing of cities, lobbing of grenades into prison cells and random execution of civilians. Other examples of state-sponsored mass murder include:
- Deliberate massacres of captives during wartime by a state's military forces, such as these committed by the Empire of Japan, the Soviet Union and Nazi Germany during the Second Sino-Japanese War and World War II: the Nanjing Massacre, the Katyn Forest Massacre of Polish citizens in 1940 and the massacres of political prisoners after the launch of Operation Barbarossa, the Three Alls Policy and the massacre of Soviet Jews at Babi Yar.
- Mass killing of civilians during total war, especially via strategic bombing, such as the Bombing of Chongqing, Bombing of Tokyo, the Blitz, the bombing of Dresden and Hamburg, or the atomic bombings of Hiroshima and Nagasaki.
- Actions in which the state caused the death of large numbers of people, which political scientist R. J. Rummel calls "democide", which, in addition to the cases above, may include man-made disasters caused by the state, such as the Holodomor in the Soviet Union, and the famines and poverty caused by the Great Leap Forward and the Cultural Revolution in the People's Republic of China.
Mass murder by individuals 
Mass murderers may fall into any of a number of categories, including killers of family, of coworkers, of students, and of random strangers. Their motives for murder vary. A notable motivation for mass murder is revenge, but other motivations are possible, including the need for attention or fame.
Examples of mass murderers include Anders Behring Breivik, Adam Lanza, Cho Seung-Hui, William Unek, Campo Elías Delgado, Jeff Weise, Woo Bum-kon, Martin Bryant, Ahmed Ibragimov, Baruch Goldstein, Robert Bales, Omar Thornton, Nidal Malik Hasan and James Holmes.
Police response and countermeasures 
Analysis of the Columbine High School massacre and other incidents where first responders waited for backup has resulted in changed recommendations regarding what bystanders and first responders should do. Average response time by police to a mass shooting is 3 minutes. In most instances that exceeds the time the shooter is engaged in killing. While immediate action may be extremely dangerous, it may save lives which would be lost if people involved in the situation remain passive, or a police response is delayed until overwhelming force can be deployed. It is recommended that civilians involved in the incident take active steps to flee, hide, or fight the shooter and that individual law enforcement officers present or first arriving at the scene attempt immediately to engage the shooter. In many instances immediate action by civilians or law enforcement has saved lives.
See also 
- Crimes against humanity
- List of rampage killers
- Mass grave
- School shooting
- Suicide attack
- War crime
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- Farhi, Paul (December 20, 2012). "Adam Lanza, and others who committed mass shootings, were white males". The Washington Post.
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- Erica Goode (April 6, 2013). "In Shift, Police Advise Taking an Active Role to Counter Mass Attacks". The New York Times. Retrieved April 7, 2013.