According to the Federal Bureau of Investigation (FBI), the general definition of spree killer is a person (or more than one person) who commits two or more murders without a cooling-off period; the lack of a cooling-off period marking the difference between a spree killer and a serial killer. The category has, however, been found to be of no real value to law enforcement, because of definitional problems relating to the concept of a "cooling-off period". Serial killers commit clearly separate murders, happening at different times. Mass murderers are defined by one incident, with no distinctive time period between the murders.
How to distinguish a spree killer from a mass murderer, or from a serial killer is subject to considerable controversy, and the term is not consistently applied, even within the academic literature. For example, The Encyclopedia of Crime and Punishment, lists five different categories of spree killers and cites Mark O. Barton as an example for the second one. He is also noted with other mass murderers, such as Patrick Sherrill, in the respective entry about mass murder. In The Anatomy Of Motive, John E. Douglas cites Charles Starkweather and Andrew Cunanan as examples of spree killers, while Jack Levin calls Starkweather a mass murderer and Cunanan a serial killer.
In Controversial Issues in Criminology, Fuller and Hickey write that "[t]he element of time involved between murderous acts is primary in the differentiation of serial, mass, and spree murderers", later elaborating that spree killers "will engage in the killing acts for days or weeks" while the "methods of murder and types of victims vary". Andrew Cunanan is given as an example for spree killing, while Charles Whitman is mentioned in connection with mass murder, and Jeffrey Dahmer with serial killing.
Douglas explains that the identity of a serial killer is generally unknown until he is caught, and the mass murderer's identity is learned only after he has committed his crime. The identity of the spree killer, on the other hand, usually becomes known by police while his spree continues, and he is sought as a fugitive.
Another term, rampage killer, has sometimes been used to describe spree killers, but it does not differentiate between mass murderers and spree killers.