British police said Friday they are investigating whether a notorious murderer has written a letter disclosing where one of his five victims was buried.

Ian Brady, dubbed the "moors murderer" for a series of killings in the 1960s, admitted in 1987 that one of his victims was Keith Bennett, a 12-year-old who disappeared in June 1964.

Brady and his lover, Myra Hindley, killed five young people in a series of sadistic murders and then buried their bodies on the moors outside Manchester.

Jackie Powell, who represents Brady as his mental health advocate, recently disclosed to the makers of an upcoming TV documentary that the killer had given her a sealed envelope addressed to the boy's mother, Winnie Johnson, to be opened after Brady's death.

Brady remains in a psychiatric hospital.

Manchester Police said Friday they have seized documents from Powell to see if they can find the letter.

Martin Bottomley, who heads Manchester's cold case unit, said it was unclear whether the report of a letter is true "or simply a ruse."

"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," Bottomley said.

In 1966, Brady was convicted of murdering John Kilbride, 12, Lesley Ann Downey, 10, and Edward Evans, 17. Myra Hindley was convicted of killing Lesley Ann Downey and Edward Evans, and shielding Brady after John's murder.

Both were sentenced to life in prison, and Hindley died in prison in 2002.

Brady, who was moved a psychiatric hospital in 1985, and Hindley admitted in 1987 that they had killed Bennett and Pauline Reade, 16. Bennett's is the only grave which has not been located.

David Kirwan, a lawyer who met Brady in 2007 on behalf of Mrs. Johnson, said he believes Brady knows where Bennett's remains are buried and could help police find them.

"However, it is important to remember who we are dealing with and how he seems to enjoy bizarre mind games and manipulation," Kirwan said.

Documentary maker Paddy Wivell said he was skeptical of Brady's motives, assuming he did hand over a letter.

"This seems to me like very much part of Brady's pathology, one of power and control," Wivell said in a BBC interview.

"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," Wivell said. "The condition that it can only be opened in the event of his death is consistent with this kind of behavior over the years."