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A serial killer is typically a person who murders three or more people,[1] usually in service of abnormal psychological gratification, with the murders taking place over more than a month and including a significant break (a "cooling off period") between them. Different authorities apply different criteria when designating serial killers; while most set a threshold of three murders, others extend it to four or lessen it to two.[3] The Federal Bureau of Investigation (FBI), for example, defines serial killing as "a series of two or more murders, committed as separate events, usually, but not always, by one offender acting alone".
Although psychological gratification is the usual motive for serial killing, and most serial killings involve sexual contact with the victim, the FBI states that the motives of serial killers can include anger, thrill-seeking, financial gain, and attention seeking The murders may be attempted or completed in a similar fashion, and the victims may have something in common: age group, appearance, gender, or race, for example.
Serial killing is not the same as mass murdering (killing numerous people in a given incident); nor is it spree killing (in which murders are committed in two or more locations, in a short time). However, cases of extended bouts of sequential killings over periods of weeks or months with no apparent "cooling off period" or "return to normalcy" have caused some experts to suggest a hybrid category of "spree-serial killer".