A mass murderer is a person who murders many people in one place and time.
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 one location where one or more people kill several others. Many acts of mass murder end with the perpetrators dying by suicide or suicide by cop. The most common incidents of mass murder include school shootings, mass shootings and bombings.
Individuals and organizations can commit mass murder whereas only one or two individuals can commit a spree killing. 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.
Mass murderers usually operate alone. Anger, resentment and malice build up in them until they explode in solitary savagery. However, excluding terrorists, there have been cases where a mass murder is committed by two or more killers. Eric Harris and Dylan Klebold are an example of a mass murderer duo.