Police are investigating whether Moors Murderer Ian Brady has written a letter to be opened on his death revealing the location of Keith Bennett's body.
But Martin Bottomley, head of investigative review of Greater Manchester Police's major and cold case crime unit, said: "We do not know if this is true or simply a ruse."
Police launched their investigation after Brady's mental health advocate told a Channel 4 documentary that he gave her a sealed envelope to pass to Winnie Johnson, the mother of his 12-year-old victim Keith Bennett, in the event of his death.
Jackie Powell, 49, who was appointed Brady's mental health advocate in 1999, was detained in south Wales on suspicion of preventing the burial of a body without lawful exercise. But detectives examining documents seized from Ms Powell's home have so far found no evidence to suggest Brady disclosed the location of Keith's body. And Ms Powell has since been released on bail pending further inquiries.
Mr Bottomley said in a statement: "We have been, and always will be, open to pursuing any new lines of inquiry that arise from significant scientific advances or credible and actionable information. I want to be explicitly clear about this: Ian Brady has not revealed to police the location of Keith's body.
"What we are looking at is the possibility, and at this stage it is only a possibility, that he has written a letter to Keith's mum Winnie Johnson which was not to be opened until after his death. We do not know if this is true or simply a ruse but we clearly have a duty to investigate such information on behalf of Keith's family."
Brady has always refused to disclose the location of Keith's body. His refusal has caused constant heartache for Keith's mother, who has cancer and recently appealed to him to break his silence before she died.
Brady and his partner Myra Hindley, who died in jail in 2002 aged 60, were responsible for the murders of five youngsters in the 1960s. Their victims were sexually tortured before being buried on Saddleworth Moor above Manchester.
Pauline Reade, 16, disappeared on July 12, 1963, and John Kilbride, 12, was snatched in November the same year. Keith Bennett was taken on June 16, 1964, after he left home to visit his grandmother. Lesley Ann Downey, 10, was lured away from a funfair on Boxing Day 1964, and Edward Evans, 17, was killed in October 1965.
Brady was jailed for life at Chester Assizes in 1966 for the murders of John, Lesley Ann and Edward. Hindley was convicted of killing Lesley Ann and Edward and shielding Brady after John's murder, and jailed for life. In 1987 the pair finally admitted killing Keith and Pauline. Both were taken back to Saddleworth Moor to help police find the remains of the missing victims but only Pauline's body was found.