Unsurprisingly, the two men credited with coming up with the term "serial killer" worked, together and separately, on some of the FBI's most gruesome cases: John Douglas and Robert K. Ressler. Their careers were so extraordinary they influenced pop culture, and at least one Oscar-winning film.

Ressler, who died in 2013 at age 76, authored several books, including a must-read for the macabre-minded: Whoever Fights Monsters: My Twenty Years Tracking Serial Killers for the FBI. He was an early adopter of criminal profiling as crime-solving method, according to NPR:

Soon after joining the FBI in 1970, Ressler had the Bureau convinced of the legitimacy of criminal profiling. Roy Hazelwood, who worked with Ressler at the FBI for more than 20 years, says that was far from his only contribution.

"He and another man, John Douglas, were the first individuals who actually conducted research on serial killers," Hazelwood says. He says together they coined the term, giving "serial killers" their namesake.

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He also consulted on The X-Files, and received a personalized clown painting from one of his most-visited subjects of study: John Wayne Gacy.

Ressler 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. On the back was an inscription that 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."

When Ressler asked what Gacy was referring to, his reply was simply: "Well, Mr. Ressler, you're the criminal profiler. You're the FBI. You figure it out."

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Douglas, who's an author and consultant, was the model for Jack Crawford, Clarice Starling's boss in The Silence of the Lambs. According to his website, named for his best-seller Mind Hunter: Inside the FBI's Elite Serial Crime Unit, he's personally studied the likes of Richard Speck, David Berkowitz, and (of course) Gacy. In addition to his work on serial killers, he's also weighed in on recent crimes, like the Amanda Knox case, and was instrumental in helping the West Memphis Three get out of jail.

So these men were probably the best-qualified people on the planet to coin a phrase describing people who can't stop killing. That said, because etymology can be a foggy realm, others have also been cited as having done so. But Ressler and Douglas are generally the most often credited.)

We'll never be able to exactly pinpoint the first serial killer, unless someone finds an incriminating cave painting. Jack the Ripper and Chicago's H. H. Holmes, both of whom operated in the 1890s, are often cited as two of the modern era's earliest. But investigative historian and author Peter Vronsky has written that the first time "serial killer" filtered into mainstream English-language usage, it was in a 1981 newspaper report describing the crimes of Atlanta child killer Wayne Williams.

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