Everyone knows the term serial killer, but probably not the man who coined it. Robert Ressler was the FBI investigator who literally wrote the book on criminology during a career spent researching serial killers and other violent offenders. Ressler died earlier this year.

NPR's Becky Sullivan tells us more.


ROBERT RESSLER: There are people that are pretty good at this. And I would consider myself one of them, certainly.

BECKY SULLIVAN, BYLINE: That's Robert Ressler talking to NPR back in 1997. By then, he had already retired from an FBI career that was both long and influential.


RESSLER: Extremely influential. The word extremely doesn't capture it.

SULLIVAN: Roy Hazelwood worked with Robert Ressler at the FBI for over 20 years. Before he joined the bureau, Ressler's time in military and civilian law enforcement had piqued his curiosity about crimes that were tough to understand: violent, sometimes sexual, always seemingly irrational. So Ressler thought by figuring out how and why these criminals worked, maybe the next time, police could better figure out who they were looking for.

Soon after joining the FBI in 1970, Robert Ressler had the bureau convinced of the legitimacy of criminal profiling. Hazelwood says that that was far from his only contribution.

ROY HAZELWOOD: He and another man, John Douglas, were the first individuals who actually conducted research on serial killers. In fact, they coined the term serial killers.

SULLIVAN: That research demanded that Ressler be a thorough and fearless investigator.

HAZELWOOD: He went on face-to-face interviews with the most notorious and successful serial killers at that particular time.

SULLIVAN: Men like Ted Bundy and Jeffrey Dahmer. And Ressler developed curious relationships with the ones he visited most often. He told a documentary team that during one of his interviews with John Wayne Gacy, the killer gifted him with a colorful self-portrait of himself as a clown.


RESSLER: There was an inscription on the back, which would read: Dear Bob Ressler, you cannot hope to enjoy the harvest without first laboring in the fields. Best wishes and good luck. Sincerely, John Wayne Gacy, June 1988. I said, well, thank you very much, John, but just what are you referring to? What harvest are we talking about? What labor are we talking about in the fields? And what fields? And he said, well, Mr. Ressler, he said, you're the criminal profiler. You're the FBI. And he said, you figure it out.

SULLIVAN: Figure what out, that's impossible to know for sure. Gacy was executed in 1994. But Robert Ressler lived much longer, long enough to see that case reopened in 2011. Investigators still hope to name the killer's remaining unidentified victims. Robert Ressler died in May. He was 76 years old.

Becky Sullivan, NPR News.

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