Police in Britain 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.
Brady and Myra Hindley murdered five children between 1963 and 1965.
The only remains not recovered are those of 12-year-old Keith, who disappeared in 1964.
But Martin Bottomley 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 Keith’s mother Winnie Johnson, in the event of his death.
Jackie Powell, 49, who was appointed Brady's mental health advocate in 1999, was detained 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 has disclosed the location of Keith's body.
Ms Powell has since been released on bail pending further inquiries.
Mr Bottomley said in a statement: "The Moors murders cast a long and dark shadow over the history of our region, but in 2009 we reluctantly concluded there was no longer any specific information to identify new search areas and the investigation to find Keith entered a dormant stage.
"However, we have always stressed this is a case we will never close. 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.
Mrs Johnson's lawyer John Ainley said: "She has remained steadfast in her resolve that Ian Brady has held the key to this tragedy.
"Over the years she has appealed directly to Brady and through me for him to reveal where Keith's body was hidden so she could at least give her son a decent burial before she died.
"Now, she herself is ill and I am deeply concerned that the knowledge of this letter and its alleged contents may have come too late to bring her any solace.
"It would be the last tragic act of a truly harrowing story."
Ms Powell told the Cutting Edge documentary: "I received a letter and a sealed envelope which said on the front 'to be opened in the event of my death'.
"He says he doesn't wish to take his secrets to the grave and within the sealed envelope is a letter to Winnie Johnson.
"Within that is the means of her possibly being able to rest."
Five children murdered in 1960s
Brady and 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 12 July 1963, and John Kilbride, 12, was snatched in November the same year.
Keith was taken on 16 June 1964, after he left home to visit his grandmother.
Lesley Ann Downey, ten, was lured away from a fun fair in December 1964, and Edward Evans, 17, was killed in October 1965.
Brady was jailed for life 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.
Brady, 74, was due to go before a mental health tribunal last month, but it was delayed after he suffered a seizure.
The tribunal was to consider his application to be transferred to a Scottish prison and be allowed to die. He has been tube-fed since refusing food 12 years ago.
It is understood that Mrs Johnson, who is being cared for in a hospice, is not yet aware of developments.
The makers of the documentary said they had not seen the envelope.
Documentaries commissioning editor and editor for Cutting Edge Emma Cooper said: "No one can verify the contents of the envelope and therefore what information it does or does not contain but, given the enormity of the implications as suggested by Brady's letter, we felt we had a responsibility to inform the family via their family liaison officer.
"Winnie's poor health was foremost in our decision-making and this is why we have decided to bring forward the broadcast of the film and make the existence of the envelope public."
Paddy Wivell, who made the documentary, questioned Brady's motives behind handing over the letter to Ms Powell.
He told BBC Radio 4's Today programme: "I think we probably have to approach it with some sense of caution too. This seems to me like very much part of Brady's pathology, one of power and control.
"He is a sadistic psychopath and it would appear that this is some sort of victory dance, a kind of power game, in the fact that he constantly puts conditions on things.
"The condition that it can only be opened in the event of his death is consistent with this kind of behaviour over the years."
The programme, Ian Brady: Endgames Of A Psychopath, will be broadcast on Monday night at 9pm.