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ABC News' Elizabeth Stuart ( @elizabethstuart) and Lauren Effron ( @leffron831) report:

The father of JonBenet Ramsey, the 6-year-old beauty pageant queen who was found dead in 1996 and whose killing has never been solved, told "Nightline" he believes his daughter's killer is still at large but that the trail has gone cold.

"He's either alive, dead or in prison, and one of those three," John Ramsey told ABC News' Juju Chang.

Since longtime police detective Lou Smit, who first investigated and then supported the Ramseys, died in 2010, Ramsey thinks authorities are out of clues in solving her case.

"It's pretty dead, I'm afraid," Ramsey said.

Watch ABC's Juju Chang's full interview with John Ramsey on " Nightline" tonight at 11:35 p.m. ET/PT

It's been 16 years since JonBenet was found dead in the basement of her family's Boulder, Colo., home the day after Christmas. That morning, the Ramsey family found a horrifying ransom note, threatening to kill JonBenet if they didn't pay $118,000.  That was the exact amount Ramsey had recently received as a bonus, leading him to believe his daughter's killing was an inside job on some level.

Ramsey, the well-to-do executive, was the one who made the shocking discovery of JonBenet's body.

"When I found her it was a rush of relief," he said. "And then of course within moments, I realized that she probably was dead.  But she was back in my arms."

PHOTOS: JonBenet Ramsey: Never-Before-Seen Pictures

For years, Boulder police suspected the Ramseys killed their daughter, but in 2008, police cleared the Ramseys of any wrongdoing and issued an apology. JonBenet's mother Patsy Ramsey died of ovarian cancer on June 24, 2006, without knowing her name had been cleared.

One of the only developments in the case, which eventually became a dead-end, was when suspect and school teacher John Mark Karr was arrested in Bangkok, Thailand, in August 2006. At the time, Karr told reporters that he was "with JonBenet when she died" but that "her death was an accident."

"He was arrested, and we were, I guess, hopeful, but not - we were a little skeptical," Ramsey recalled. "What I realized was that this is going to be difficult. He didn't look like a killer. He looked like a nice guy. He was a teacher in an elementary school. I thought, 'Boy, that's not what I would have expected.' I expected a monster."

Karr was jailed briefly, but Boulder prosecutors soon abandoned their case against him when DNA tests failed to link Karr to DNA found in JonBenet's underwear, disproving his claim.

In his newly released book, "The Other Side of Suffering," John Ramsey talks about his incredible journey from grief to hope.  He recounts how deeply his faith helped him navigate course from suffering to forgiveness.

"I don't know for sure what happened, and I don't want to know, really," John Ramsey said.