According to the working FBI website (and a direct quote), it is two or more.
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A '''serial killer''' is a murderer who is responsible for multiple killings carried out on separate occasions. Definition from the FBI website: ''Serial Murder: The unlawful killing of three or more victims by the same offender(s), in separate events.''
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A '''serial killer''' is a murderer who is responsible for multiple killings carried out on separate occasions. Definition from the FBI website: ''Serial Murder: The unlawful killing of two or more victims by the same offender(s), in separate events.''<ref>[https://www.fbi.gov/stats-services/publications/serial-murder#two Serial Murder -- II. Definition of Serial Murder]. ''FBI.gov''. Retrieved April 27, 2018.</ref>