A serial killer (also called a serial murderer) is typically a person who murders three or more persons, with the murders taking place over more than a month and including a significant period of time between them. While most authorities set a threshold of three murders, others extend it to four or lessen it to two.Psychological gratification is the usual motive for serial killing, and many serial murders involve sexual contact with the victim. The Federal Bureau of Investigation (FBI) states that the motives of serial killers can include anger, thrill-seeking, financial gain, and attention seeking, and killings may be executed as such. The victims may have something in common; for example, demographic profile, appearance, gender or race. Often the FBI will focus on a particular pattern serial killers follow. Based on this pattern, this will give key clues into finding the killer along with their motives.Although a serial killer is a distinct classification that differs from that of a mass murderer, spree killer, or contract killer, there exist conceptual overlaps between them. Some debate exists on the specific criteria for each category, especially with regard to the distinction between spree killers and serial killers. Source: Wikipedia (en)