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 period of time between them.[1][2] Different authorities apply different criteria when designating serial killers.[3] For example, while most authorities set a threshold of three murders,[1] others extend it to four or lessen it to two.[3] The Federal Bureau of Investigation (FBI) defines serial killing as "a series of two or more murders, committed as separate events, usually, but not always, by one offender acting alone".[4]