Serial killer Rose West has been urged to reveal what happened to a schoolgirl who vanished in Gloucester.
Mary Bastholm was 15 when she went missing on her way to catch a bus to visit her boyfriend in the city in 1968.
Her brother Peter Bastholm - speaking ahead of the 20th anniversary of the Cromwell Street murder investigation - has called on West to divulge what happened to his sister.
Mr Bastholm, 69, believes Mary was an early victim of West's builder husband Fred West, who killed himself in prison a year after his arrest.
Mary's body has never been found.
Speaking to Sky News, for Fred & Rose: the unanswered questions, Mr Bastholm said West was probably the "only one" who knew about his sister's disappearance.
"I think Rose knows exactly what happened to Mary because I assume she and Fred discussed it between themselves. Whether she will ever disclose it, I really don't know," Mr Bastholm told the programme.
"It would be a final act to find Mary's body and lay it to rest after all these years, after all we have gone through."
Mary vanished in 1968 - a time when Fred West, who often did jobs in the street where she worked as a waitress in a cafe, was abducting girls in Gloucester.
Her body was not found during the excavation of the West's home, now known as the House of Horrors on 25 Cromwell Street, Gloucester, which began on February 24, 1994.
However, police investigating the murders did find evidence that suggested a link between Mary and Fred West.
Mr Bastholm told Sky News: "My wife and I went down to the police station to view a lot of things that had been taken from the Wests' house.
"My wife picked out a silver locket on a chain and she said that was Mary's because she wore it as a bridesmaid at our wedding.
"We had a photo of it and police took it away to examine, but because it was a black and white picture they said they could not tell if the locket was silver or gold, so nothing came of it. But it was exactly like the one found at Cromwell Street."
Fred West is also said to have confessed to killing Mary in conversation with his son, Stephen, but never admitted it to police.
Tony Butler, former chief constable of Gloucestershire Police, told Sky News: "I don't know if Rose West will feel at some point that she has some moral obligation to help the relatives of people, of other victims, I don't know."
But her solicitor, Leo Goatley, told the programme he believed West would reveal what happened to Mary if she knew.
"If Rose West was asked now about what happened to Mary Bastholm, I think if she knew, she would say so," Mr Goatley said.
The Wests tortured, raped and murdered an unknown number of women over a 20-year period, between 1967 and 1987.
Fred West was charged with 12 murders but committed suicide in a Birmingham prison before his trial. Rose West was convicted of 10 murders in November 1995 and is serving a life sentence.
Fred & Rose: the unanswered questions will be shown on Sky News at 9.30pm tonight.