America's foremost criminal profiler is now giving his insight into the mystery of who killed two Delphi, Indiana, teens.

Abby Williams and Libby German were found dead after they went for a hike in February 2017 and never returned. More than two years later, authorities have yet to catch their killer.

Among the few clues? Video from Libby's cellphone showing a man walking toward the two girls and a recording in which a man apparently says "guys, down the hill."

Former FBI Special Agent John Douglas, aka the Mindhunter, told Inside Edition the audio, of which an extended clip was only recently released, may hold a vital clue, specifically the use of the word "guys."

"Guys says to me that maybe he may know these victims," said Douglas. "Or it's a person ... someone who has interaction with children."

He added that the person who killed the two likely has some kind of "criminal history."

"You don't wake up one day and commit a double homicide like this," he said. "There has to be some kind of trail."

Douglas, who inspired Jonathan Groff's character on the Netflix series "Mindhunter," also addressed the recent, and chilling, suggestion by police that the killer might have been present at a news conference last month in which the extended clip was released, together with the video of the suspect and a new police sketch.

"That tactic, if they're going to do it, they should have done it two years ago," Douglas said. "They should also be checking and looking to see people who were here, now they came up with some reason to leave the area."

Authorities said Monday the release of the new evidence has triggered a wave of about 3,000 fresh tips.

Douglas' new book, "The Killer Across the Table," spotlights the most diabolical killers he has ever encountered.

You can read an excerpt from the book here.

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