Paedophile Robert Howard admitted in prison that he killed schoolgirl Arlene Arkinson , a coroner's court has been told.
The sex predator was overhead boasting about getting away with murder while he was being held at a jail in England, it was revealed.
Howard said in 2009: "On my first, I know I got away with it." A year later, he was overheard saying: "I thought I got away with it ... I did."
The revelations emerged as a top detective who led the Arkinson inquiry for nine years gave evidence to Belfast Coroner's Court.
Chief Superintendent Raymond Murray said the conversational snippets had formed a vital part of his investigation.
Mr Murray said: "This was an implicit admission at the very least, if not an explicit one, that he had got away with murder.
"The only murder that he had ever been acquitted of in a court of law, that we know of, was Arlene Arkinson."
Fifteen-year-old Arlene, from Castlederg, Co Tyrone, vanished after a night-out across the border in Co Donegal.
She was last seen being driven off during the early hours of August 14, 1994, down a country road with the convicted child killer. Her body has never been found.
Chilling conversational extracts in which Howard appeared to recount murder, clean-up and disposal of a body were also read to the inquest.
They included talk of drugging, choking and a blood-soaked body. He also mentioned "burning her".
Howard was quoted as saying : "See, I couldn't hold her ... I got her over to the sink that's just inside.
He added: "She's strong" and "I bound her".
The conversations were reported to police over a three-year period between 2009 and 2012, although the source of the information was not disclosed to the court.
At the time, Howard was serving a life sentence for the murder of South London teenager Hannah Williams.
He further stated: "She was covered ... feeling really guilty ... it can last forever, covered in blood ... violence."
At one point Coroner Brian Sherrard halted proceedings when Kathleen Arkinson, the missing schoolgirl's sister, broke down in tears.
In some discussions, Howard appeared to relive the killing.
"I'll stick you in the f***ing ground ... I can't be done again ... Jesus ... what were you doing back up in the .... you shouldn't have done," he said.
The talk also referenced a sex attack.
"And on the drive .... I f***** her."
Regarding the disposal of a body, he stated: "There was a dead body in the back of me van, unfortunately there was." On another occasion he said: "Body was in the boot."
In 2005, Howard was found not guilty of murdering Arlene by a jury which was never told of his lengthy criminal history.
He remained the prime suspect in the high-profile case until his death in prison last year aged 71.
The court was told he had expressed concerns about double jeopardy and was overheard claiming: "It doesn't matter because I was found not guilty but now they resurrect the case."
Although police believed the prison conversations were hugely significant, prosecutors did not believe they met the high evidential threshold to overturn the acquittal and commence a re-trial.
They did, however, reinvigorate the investigation and prompted a three-week forensic examination of Howard's former flat on Main Street in Castlederg which unearthed a number of new leads.
"The nature of the conversations clearly indicate that somebody is recollecting the actions of killing and the resulting clean-up and disposal of the body," said Mr Murray.
"The information from the conversational extracts indicated that the maisonette at 49A Main Street, Castlederg, was the locus of the attack."
Mr Murray was responsible for overseeing the Arkinson investigation from October 2005 until early 2014 - longer than any other senior officer.
Although he did not personally interview Howard, the detective was present when he was quizzed in prison, describing him as a cunning character
"There is no doubt there was a shrewdness although not the academic ability to back it up," he said.
Mr Murray also acknowledged relations with the Arkinson family had, at times, been difficult, adding: "It is fair to say that relations with the family were tense and strained."
The scope of the long-running inquest is to examine whether Arlene Arkinson is dead, how, when and where she died.
Mr Murray said the evidence suggested she had been murdered by Howard.
"It is my firm belief that Arlene Arkinson is dead," he told the court.
"Given the totality of the information, I believe she was murdered during the early hours of August 14, 1994 and the location of the murder was 49a Main Street, Castlederg, and the person that murdered her was Robert Howard and he murdered her in a sexually-motivated attack."
The case has been adjourned.