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.