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Robert K. Ressler (February 21, 1937 – May 5, 2013) was a former FBI agent and author. He played a significant role in the psychological profiling of violent offenders in the 1970s and is often credited with coining the English term "serial killer".
Ressler joined the FBI in 1970 and was recruited into the Behavioral Science Unit that deals with drawing up psychological profiles of violent offenders who typically select victims at random, such as rapists and serial killers.

In the early 1980s, Ressler helped to organize the interviews of thirty-six incarcerated serial killers in order to find parallels between such criminals' backgrounds and motives. He was also instrumental in setting up Vi-CAP (Violent Criminal Apprehension Program). This consists of a centralized computer database of information on unsolved homicides. Information is gathered from local police forces and cross-referenced with other unsolved killings across the United States. Working on the basis that most serial killers claim similar victims with a standard method (modus operandi) it hopes to spot early on when a killer is carrying out crimes in different jurisdictions. This was primarily a response to the appearance of nomadic killers who committed crimes in different areas. So long as the killer kept on the move, the police forces in each state would be unaware that there were multiple victims and would just be investigating a single homicide each, unaware that other police forces had similar crimes. Vi-CAP would help individual police forces determine if they were hunting for the same perpetrator so that they could share and correlate information with one another, increasing their chances of identifying a suspect.

He worked on many cases of serial homicide such as Jeffrey Dahmer, Richard Chase and John Joubert.

A serial killer is typically a person who murders two or more people, usually in service of abnormal psychological gratification, with the murders taking place over more than a month and including a significant break (a "cooling off period") between them.

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