Ms Johnson first wrote to Hindley over 20 years after his disappearance
Moors murderer Myra Hindley broke down in tears when she received a letter from the mother of one of her victims, it has been revealed.
Winifred Johnson's 12-year-old son Keith Bennett disappeared in 1964. She wrote to Hindley about 20 years later.
The news comes as previously unseen documents in the National Archives at Kew were released.
Ms Johnson wrote: "I am on bended knees begging you to end this torture and finally put my mind at rest."
Hindley and her lover Ian Brady were convicted of killing children in 1966.
She was found guilty of murdering Lesley Ann Downey, aged 10, and 17-year-old Edward Evans, as well as being an accessory to the murder of 12-year-old John Kilbride.
The killings became known as the Moors Murders because their victims' bodies were buried on Saddleworth Moor near Manchester.
Keith Bennett was last seen on his way to his grandmother's house
Keith Bennett was last seen by his mother on 16 June, as he made his way to his grandmother's home.
In 1987 - after Mrs Johnson sent her first heartfelt plea - Hindley and Brady confessed to killing Keith Bennett and Pauline Reade, aged 16.
Police searches of the location provided by Brady and Hindley led to the discovery of Pauline Reade's body but Keith's was never found.
In the neat, handwritten letter dated 31 October, 1986, Mrs Johnson explained who she was and appealed to Hindley as a fellow woman for help.
She described the nightmare she had endured since she last saw her son in 1964.
"Not knowing whether my son is alive or dead, whether he ran away or was taken away, is literally a living hell, something which you no doubt have experienced during your many, many years locked in prison," she wrote.
Mrs Johnson said it had taken her five weeks to write the letter "because it is so important to me that it is understood by you for what it is, a plea for help".
A memo from staff at Cookham Wood Prison in Rochester, Kent, suggested they believed Mrs Johnson had contacted Hindley for the first time as a result of a book by Fred Harrison on the Moors Murderers, which "strongly implies" that Brady and Hindley were involved in Keith Bennett's disappearance.
The memo, in which the author's name is obscured, described Hindley's reaction to the letter.
It said: "She became extremely upset and tearful whilst reading it and it took a very long time for her to compose herself sufficiently to talk (this is most unusual as Myra is normally very controlled)."
Hindley denied all knowledge of Keith Bennett's death when she first read the letter, according to the memo - which also referred to Hindley's dilemma about how to respond because of fears of newspapers publishing her reply or being seen as callous if she did not reply.
Officers still believe they can find Keith Bennett's body and have been "actively" searching for it for the last three years, police said.
The force said it was still "their goal" to find his body.
Brady is serving his sentence at Ashworth Hospital on Merseyside.
Hindley died in prison six years ago.