The victims do not usually know each other, and they do not know the killer. They are connected by the desires of a man they do not even know. The serial killer usually has at least 3 victims. The murders are often very brutal, involving some kind of ritual that has specific meaning only for the offender. Sexual violence is usually a factor, whether it be before or after death.
Some killers can only get sexual satisfaction through the pain of their victim.
Other problems arise in developing a definition for serial killers. For example, would a hit man be a serial killer - they often have numerous victims, but they kill for money. In 1978, the FBI set up a Behavioural Science Service Unit in Quantico, Virginia. At this centre, serial killers are studied, and profiles are compiled on suspects. This information in turn allows the FBI to do profiles of cases that are brought to their attention.