When Ressler joined the FBI, profiling and the psychology of criminals weren't used in investigations. Instead, detectives relied on concrete pieces of evidence to solve cases. Still, Ressler believed that criminal profiling would be a big help, especially in cases that involved serial killers (via Crime Traveller). Ressler supervised a team in the Behavioral Science Unit in the 1970s, and it was at that time when he conducted serial killer interviews.
Ressler interviewed 36 murderers and violent offenders, some of which include Ed Kemper, John Wayne Gacy, Jeffrey Dahmer, and Ted Bundy, just to name a few. The insights that he gained over the years allowed him to help develop the Violent Criminal Apprehension Program (Vi-CAP), which is a database that allows investigators to distinguish patterns in homicides, as reported by Refinery 29. According to Ressler, he was in awe of serial killers and the interviews allowed him and his colleagues to help catch future offenders. "I am somewhat in awe of their ability to get away with their crimes for so long. It becomes a challenge for me, and people like me, to stop that cycle. You're not really admiring them as much as you are in awe that they are able to commit these crimes in sequence over a period of time," he stated in an interview via Sci-Fi Online.
