The lawyer for Winnie Johnson, John Ainley, has said that his client's family is treating recent developments "with some suspicion".
He told ITV News it was "very timely" that the documentary which brought the letter claim to light is due to be aired tomorrow.
Police first became aware of the possible existence of a letter from Ian Brady his victim's mother in late July, ITV News can reveal
Keith Bennett's brother speak out over body claims
Jackie Powell was detained in south Wales on suspicion of preventing the burial of a body without lawful exercise.