Robert Ressler played a significant role in the psychological profiling of violent offenders in the 1970's as an FBI agent. Ressler was recruited into the FBI's Behavioral Analysis Unit, which focuses on the psyche of violent offenders, such a rapists and murders. Their job was to develop psychological profiles on offenders who typically chose their victims at random.
Ressler interviewed thirty-six incarcerated serial killers trying to establish a pattern of murders being committed in other jurisdictions. This led to cross-referencing unsolved murders in other states. He suggested that when considering a killers standard method and victim type, they could spot early on when a killer is carrying out crimes in other jurisdictions.
Some of Ressler's notable cases include Jeffrey Dahmer, Richard Chase and John Joubert.
Ressler retired from the FBI in 1990 and authored a number of books about serial murder.