London (CNN) -- Decades after the crime, police are investigating what could be a clue to the burial place of the last unrecovered victim of one of Britain's most notorious killers, "Moors Murderer" Ian Brady.
Keith Bennett was just 12 years old when he went missing on June 16, 1964, in northern England on his way to visit his grandmother.
Brady, who was jailed for life in 1966 for three other child murders committed with the help of his partner Myra Hindley, admitted killing the boy in 1987 but has never disclosed the whereabouts of his body.
The other four victims were buried on Saddleworth Moor, a lonely upland expanse in the Pennine hills, near Manchester.
Keith's mother, Winnie Johnson, has spent the decades since he vanished searching for her lost son but is herself gravely ill.
Now, police are investigating claims that Brady may recently have given details of the burial spot to one of his longtime visitors, in a sealed letter to be passed to Johnson after Brady's death.
The information came to light when a documentary filmmaker interviewed a woman who has been acting as Brady's mental health adviser, the Press Association news agency reported.
A 49-year-old woman was arrested in South Wales on Thursday on suspicion of preventing the burial of a body without lawful exercise, and has been released on bail pending further inquiries, a Greater Manchester Police statement said.
Documents have also been seized from the address and are being examined, police said.
In the statement, Martin Bottomley, head of the force's cold case review team, cautioned against leaping to conclusions but said police were taking the lead seriously.
"I want to be explicitly clear about this: Ian Brady has not revealed to police the location of Keith's body," he said.
"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.
"Since Keith was so cruelly taken away all those years ago, our thoughts have always been with his family as they try to come to terms with what happened. All they want is to finally be able to lay Keith to rest."
The Moors murders have cast "a long and dark shadow" over the area, he said.
Police have been in contact with Keith Bennett's family and staff at Ashworth Hospital, the high-security psychiatric institution where Brady is held.
He has refused food for the past 12 years, in a bid to die, and is fed through a tube, the Press Association reports.
Brady and Hindley were taken by police to Saddleworth Moor to look for the bodies of Bennett and another victim, 16-year-old Pauline Reade, after they admitted those killings in 1987, the news agency says. However, only Reade's remains were found.
Keith's brother, Alan Bennett, runs the family's Searching for Keith website, where information about the case is posted, as well as pictures of the family taken before the tragedy.
"He was a son, a brother, a friend and should have been a father, grandfather and maybe a great-grandfather. He should have been whatever he wanted to be," Alan Bennett writes.
A police poster appealing for information that was put up in shops and windows at the time Keith went missing shows a cheerful, bespectacled boy.
Johnson is quoted on the website as saying: "My greatest hope in this life from the day he left is to have him home, I now accept he is not alive but I still believe I have the right as his mother to give him a Christian burial."