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 people 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. 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.