A serial killer is a person who murders three or more people, with the killings taking place over a significant period of time. The serial killers' psychological gratification is the motivation for the killings, and many serial murders involve sexual contact with the victims at different points during the murder process. 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 tend to have things in common such as, demographic profile, appearance, gender or race. Although a serial killer is a distinct classification that differs from that of a mass murderer, spree killer, or contract killer, there are overlaps between them.