The lawyer for Winnie Johnson, the mother of Keith Bennet, the 12-year-old killed by Moors murderer Ian Brady, said his client's family is treating recent developments "with some suspicion".
John Ainley said: "I think they've had their hopes raised so many times and those have been dashed. They feel that until there is very positive information and the police have carried out their investigation, and there is a letter and that letter contains important information, I think they will remain skeptical."
It follows claims that Brady is believed to have revealed details about where his body is buried.
Greater Manchester Police have released a statement saying the killer had finally disclosed details of the location of the body to one of his long-term visitors at the high-security hospital where he is being detained.
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.
While Brady has revealed where some of his victims were buried, until now, he has always refused to disclose any details about the location of Keith's body, despite pleas from his family.
His refusal has caused constant heartache for Keith's mother, Winnie, who has cancer and recently appealed for Brady to break his silence 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.
They lured the children and teenagers to their deaths, with victims sexually tortured before being buried on Saddleworth Moor above Manchester.