Moors murderer Ian Brady is believed to have revealed details about where 12-year-old victim Keith Bennett is buried.
Greater Manchester Police said in a statement that Glasgow-born Brady had recently given details of the location of the body to one of his long-term visitors.
The force added that Jackie Powell, 49, who was appointed Brady's mental health advocate in 1999, was detained in south Wales on suspicion of preventing the burial of a body without lawful exercise.
A Greater Manchester Police spokesman said: "On 30 July 2012, Greater Manchester Police received information that led officers to believe that Ian Brady had recently given details of the location of Keith Bennett's body to one of his long-term visitors. Officers in the cold case unit made inquiries to accuracy of this report and have since had extensive contact with Ashworth hospital and Keith's close family.
"As a result, warrants obtained under the Police and Criminal Evidence Act were executed at Ashworth hospital and a private address in south Wales today.
"A 49-year-old woman has been arrested in south Wales on suspicion of preventing the burial of a body without lawful exercise and remains in police custody for questioning. Searches of both locations are ongoing."
Brady and his partner, Myra Hindley, were responsible for the murders of five youngsters in the 1960s. They lured children and teenagers to their deaths, with victims sexually tortured before being buried on Saddleworth Moor above Manchester.
Pauline Reade, 16, disappeared on her way to a disco on July 12 1963 and John Kilbride, 12, was snatched in November the same year.
Keith Bennett was snatched on June 16 1964 after he left home to visit his grandmother; Lesley Ann Downey, 10, was lured away from a funfair on Boxing Day 1964; and Edward Evans, 17, was killed in October 1965. Brady was given life at Chester Assizes in 1966 for the murders of John, Lesley Ann and Edward.
Hindley was convicted of killing Lesley Ann and Edward and shielding Brady after John's murder, and jailed for life. In 1987 the pair finally admitted killing Keith and Pauline. Both were taken back to Saddleworth Moor in 1987 to help police find the remains of the missing victims but only Pauline's body was found.
Keith's mother Winnie Johnson has made repeated calls for Brady to reveal the location of his grave. Last December Mrs Johnson said she wanted to hear face-to-face from Brady where her son was buried.
Hindley died in jail in November 2002, aged 60. Brady has spent the last 25 years at the high-security Ashworth Hospital in Merseyside.