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Note: Varies by jurisdiction
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Mass murder is the act of murdering a number of people, typically simultaneously or over a relatively short period of time and in close geographic proximity.   The FBI defines mass murder as murdering four or more persons during an event with no "cooling-off period" between the murders. A mass murder typically occurs in a single location where one or more people kill several others.   Many acts of mass murder end with the perpetrator(s) dying by suicide or suicide by cop[ citation needed].
A mass murder may be committed by individuals or organizations whereas a spree killing is committed by one or two individuals. 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 may kill people over long periods of time. Mass murder is the hypernym of genocide, which requires additional criteria.
Mass murder is also defined as the intentional and indiscriminate murder of a large number of people by government agents; for example, shooting unarmed protesters, throwing grenades into prison cells, and randomly executing 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 because of dislike or intolerance. 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.
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 protesters, lobbing of grenades into prison cells, and random execution of civilians. Other examples of state-sponsored mass murder include:
- Deliberate massacres of captives or civilians during wartime or periods of civil unrest by the state's military forces, such as those committed by Genghis Khan, the Golden Horde, the troops of Vlad the Impaler, the Empire of Japan, the Soviet Union, and Nazi Germany during the Second Sino-Japanese War and World War II, the Nanjing Massacre, the mass killings in Indonesia during Suharto's rise to power,  the murder of suspected leftists during Operation Condor in South America,  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, the massacre of Soviet Jews at Babi Yar, the mass murder of the Hungarian, Serbian and German population in Vojvodina in the "Vengeance of Bacska", the murder of 24 unarmed villagers by British troops in the Batang Kali massacre during the Malayan Emergency, the murder of Vietnamese civilians by American soldiers in the My Lai Massacre during the Vietnam War, the genocidal massacres of the Maya population during the Guatemalan Civil War,  repeated attacks on civilians during the Syrian Civil War including the Al-Qubeir massacre.
- 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,  the Khmer Rouge years in Cambodia, and the famines and poverty caused by the Great Leap Forward and the Cultural Revolution in the People's Republic of China. 
Many terrorist groups in recent times have used the tactic of killing many victims to fulfill their political aims. Such incidents have included the Beirut barracks bombings in October 1983 by IJO, the Başbağlar attack by the PKK in 1993, the September 11 attacks in September 2001 and the 2004 Madrid train bombings in May 2004 by Al-Qaeda, and the November 2015 Paris attacks in November 2015 by ISIL.
Certain cults, especially religious cults, have committed a number of mass killings and mass murder-suicides. These include Jim Jones' Peoples Temple in Jonestown, Guyana, where 919 people died in 1978; David Koresh's Branch Davidians in Waco, Texas, where 87 died in 1993; the Order of the Solar Temple in Canada, Switzerland, and France, where 75 died in 1994, 1995, and 1997; Shoko Asahara's Aum Shinrikyo, which killed 12 in Tokyo, Japan, in 1995; Marshall Applewhite's Heaven's Gate in San Diego, California, where 39 died in 1997; and the Movement for the Restoration of the Ten Commandments of God in Uganda, where 778 died in 2000.
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 King Dipendra of Nepal,  Mohamed Lahouaiej-Bouhlel, Anders Behring Breivik, Timothy McVeigh, Jack Gilbert Graham, Adam Lanza, Seung-Hui Cho, Eric Harris and Dylan Klebold, Robert Steinhäuser, Pekka-Eric Auvinen, Matti Juhani Saari, Thomas Watt Hamilton, Tim Kretschmer, Marc Lépine, Aaron Alexis, William Unek, Campo Elías Delgado, Jeff Weise, Michael McLendon, Woo Bum-kon, Martin Bryant, Ahmed Ibragimov, Baruch Goldstein, Robert Bales, Jared Lee Loughner, Omar Thornton, James Huberty, Andrew Kehoe, Christopher Harper-Mercer, George Hennard, Stephen Paddock, Omar Mateen, Nidal Malik Hasan, James Holmes, Dylann Roof,  Andreas Lubitz,  and Elliot Rodger. 
Acting on the orders of Joseph Stalin, Vasili Blokhin's war crime killing of 7,000 Polish prisoners of war, shot in 28 days, is notable as 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. 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 may 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. 
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 "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." 
How such rarely occurring incidents of homicide are classified tends to change significantly with time. "In the 1960s and 1970s,... it was understood that the key feature of [a number of such] cases was a high body count. These early discussions of mass murder lumped together [a variety of] cases that varied along what would come to be seen as important dimensions:
- Time: Did the killings occur more or less simultaneously, or did they extend over several days, months, or years?
- Place: Did the killings occur in a single location, or in a variety of places?
- Method: How were the victims killed?" 
In the late decades of the 20th century and early years of the 2000s, the most popular classifications moved to include method, time and place. While such classifications may assist in gaining human meaning, as human-selected categories, they can also carry significant meaning and reflect a particular point of view of the commentator who assigned the descriptor. 
MASS MURDER INFORMATION
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