Detectives are examining claims Moors Murderer Ian Brady may have revealed information about where 12-year-old victim Keith Bennett is buried.
Police are investigating whether Brady, 74, gave details to his mental health advocate, Jackie Powell.
Ms Powell was arrested in south Wales on suspicion of preventing a lawful burial and has been bailed.
Police have searched the hospital where Brady is being treated but no relevant documents were found.
Detectives checking documents seized from Ms Powell's home have so far found no letter or any evidence to suggest Brady has disclosed the location of Keith's body.
Brady and Myra Hindley murdered five children between 1963 and 1965.
The bodies of three of them were buried on Saddleworth Moor near Manchester, while one was found at Brady's home.
Keith's is the only body never found.
He was abducted on his way to visit his grandmother in Manchester on 16 June 1964. His mother Winnie Johnson, 78, has fought a long campaign to get Brady to reveal the location of her son's grave.
A Greater Manchester Police (GMP) spokesman said: "On 30 July 2012, Greater Manchester Police received information that led officers to believe that Ian Brady had recently given details of the location of Keith Bennett's body to one of his long-term visitors."
'Duty to investigate'
Martin Bottomley, from Greater Manchester Police's Major and Cold Case Crime Unit, 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."
The police were alerted to the possibility of information about the location of Keith's remains by the makers of a Channel 4 documentary about Brady, who said Ms Powell had told them she had received letters from the killer, one of which was for Mrs Johnson.
In the programme, Ms Powell said that "clearly there's something within the letter that may be able to find her son, I would suggest".
She added that she thought there "might be nothing in the letter" and, as a result, she had been "in a dilemma" as to whether to inform police.
Sky News reported Ms Powell told a reporter that her words had been misrepresented and she was seeking legal advice.
The documentary's editor Emma Cooper said, after learning of the documents, she had informed police.
"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," she said.
In a statement posted on a site dedicated to Keith, his brother Alan Bennett said that "until some definite information is found regarding Keith's whereabouts we do not want to raise our hopes too high at this stage".
"Nonetheless, it is a very important development," he added.
Brady, 74, was jailed for life in 1966 at Chester Assizes for the murders of Lesley Ann Downey, 10, John Kilbride, 12, and Edward Evans, 17.
His victims were kidnapped and tortured.
He has spent the past 25 years at the high-security Ashworth Hospital in Merseyside and has been tube-fed since refusing food 12 years ago.
Hindley, who died in prison in November 2002 aged 60, was given a life sentence for the murders of Lesley Ann and Edward and for shielding Brady after John's murder.
In 1987 they admitted killing Keith and 16-year-old Pauline Reade, whose bodies had not been found.
Both were taken back to Saddleworth Moor to help police find the remains of the missing victims but only Pauline's body was found.
'Proper family burial'
The director of public prosecutions at the time decided prosecuting Brady and Hindley for the final two killings would not be in the public interest.
Police abandoned the hunt for Keith's body in 2009 and officers said they would need fresh evidence to resume the search.
Greater Manchester Police said warrants were executed at Ashworth Hospital, where Brady is being treated, and an address in south Wales on Thursday.
Last month Brady, who was born in Glasgow, was due to go before a mental health tribunal to consider his application to be transferred to a Scottish prison and be allowed to die. But the hearing was postponed when he suffered a seizure.
Last December, Mrs Johnson, from Longsight, Manchester, said she wanted to attend the planned hearing to face Brady, saying she had "never seen him face to face".
But in the days leading up to the tribunal, before it was called off, she said it would be too traumatic for her to attend.
Mrs Johnson's solicitor John Ainley said his client had "always believed that Ian Brady knew where her son was buried".
"Her one and only desire is to give her son a proper family burial during her lifetime."
He added he found it "difficult to believe this story is true, but if it is, she will be bitterly disappointed that after all the requests she has made to Brady, that he has left it to this very late stage to provide information, and even that information isn't readily available.
"I remain to be convinced - it seems rather late in the day and a strange turn of events to come to light at this time."