The term mass murder refers to when a murder of four people or more happens by the same person or group at the same time. Usually the murders happen at the same time or over a relatively short period of time. The FBI defines mass murder as the killing of many people without any cooling-off period. By comparison a serial killer has a cooling-off period between murders. Murder can be done by a person, a gang, a terrorist group, a government or police. Mass murderers commit their crime for many reasons. Examples of why they do it include: politics/religion/ethnicity, sadism, being bullied, mental disorders and inceldom. In many cases, the killer commits suicide or is killed by police.
The University of Texas massacre in August 1966, the 1999 Columbine High School massacre, the September 11 attacks, the Virginia Tech massacre, the 2012 Aurora shooting and the 2012 Sandy Hook Elementary School shooting are some examples of mass murders that have happened in the United States.
The United Kingdom's mass murders include the Denmark Place fire in London in 1980, the Hungerford massacre in Hungerford, Berkshire in 1987 and the Pan Am Flight 103 Lockerbie, Scotland in 1988. Australia's mass murders include the Port Arthur massacre in 1996. France's include the November 2015 Paris attacks. Turkey's include the 2015 Ankara bombings.
- "Serial Murders". The Federal Bureau of Investigation. Retrieved Feb 4, 2014. CS1 maint: discouraged parameter (link)
- Lee Mellor, Rampage: Canadian Mass Murder and Spree Killing (Toronto: Dundurn Press, 2013), p. 15
- Ling, Justin et al 2018). "The 'incel' community and the dark side of the Internet". The Globe and Mail. 
- Nagle, Angela. "The New Man of 4chan." The Baffler 30 (2016): 64-76.
- "School Shooting: 28 dead". The New York Times. Retrieved Feb 4, 2014. CS1 maint: discouraged parameter (link)