Ian Brady's mental health advocate has told police she gave a letter in which he apparently reveals the location of one of his victims back to him, sources said on Friday night.
Police launched an investigation after Jackie Powell told a Channel 4 documentary the Moors murderer gave her a sealed envelope to pass to Winnie Johnson, mother of 12-year-old Keith Bennett, in the event of his death.
The TV production team passed on the information to Greater Manchester Police on July 30 and Ms Powell, 49, who was appointed Brady's mental health advocate in 1999, was detained yesterday on suspicion of preventing the burial of a body without lawful exercise.
A privately funded search on the rocky moors in 2010 failed to find Keith's body
'Moors Murderers' Ian Brady and Myra Hindley
Keith's mother Winnie is very unwell. Her son's body has been missing for over 47 years
But police sources said Ms Powell claims she returned the envelope to Brady at some point between July 30 and her arrest.
Detectives examining a large number of documents seized from Ms Powell's home in south Wales have so far found no letter or any evidence to suggest Brady has disclosed the location of Keith's body.
A search of Brady's cell at Ashworth Hospital in Merseyside has also failed to uncover the alleged letter.
Ms Powell has been released on bail pending further inquiries.
It is thought that Ms Powell operates on a freelance basis.
Ashworth, where Brady has been detained since he was declared insane, confirmed she was not employed by the hospital or by the independent mental health advocacy service provided there.
Martin Bottomley, head of investigative review at Greater Manchester Police's major and cold case crime unit, said his officers were trying to establish if the letter exists or is "simply a ruse".
He said: "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."
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Brady, 74, has always refused to disclose the location of Keith's body.
His refusal has caused constant heartache for Mrs Johnson, who has cancer, and recently appealed to him to break his silence before she died. She is receiving care in a hospice and is said to be unaware of today's developments.
Her lawyer, John Ainley, a senior partner at Oldham-based North Halliwell Ainley, said he was "sceptical" about the possibility that Brady has revealed the location of Keith's body.
"She has remained steadfast in her resolve that Ian Brady has held the key to this tragedy," he said.
"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."
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.