Police searching a cafe in Gloucester for one of Fred West's possible victims have said they may interview his widow, Rose, about the case.
Detectives believe the body of Mary Bastholm, a schoolgirl, could be buried within a void beneath the Clean Plate cafe just half a mile from where the Wests carried out a string of murders at their home in Cromwell Street.
As well as identifying six possible burial sites under the cellar floor, investigators are examining a scrap of blue material identified at the scene that appears to match the coat the 15-year-old was wearing when she disappeared in January 1968.
Detectives said they had not ruled out visiting Rose West in her cell at HMP New Hall in West Yorkshire to question her about the case in the hope of bringing some closure to Mary's family.
The teenager was last seen waiting to catch a bus more than 53-years ago and it has long been feared that serial killer West may have stopped to give her a lift before murdering her.
She worked at the cafe - then called the Pop In - around the time of her disappearance. West was a regular there in the late 1960s.
It is also thought he carried out building work including laying the concrete floor in the cellar shortly after Mary was reported missing.
West did not meet his future wife until 1969, the year after Mary's disappearance, but detectives believe she may still have vital information about the case.
The latest developments came after a documentary film crew working on film about the Wests began examining the cellar beneath the cafe.
They passed their findings to detectives who ordered a forensic archaeological survey to take place.
Gloucestershire Police said six separate voids had been located under the floor and it had given the green light for further excavations to take place.
Det Chief Insp John Turner, who is leading the investigation, said: "What we are going to do is concentrate on the excavation within the basement.
"As and when we know more, I will then consider whether people need to be interviewed and spoken to and Rose West may be one of those people."
The Wests' eldest son, Stephen, has maintained his father told him shortly before his death that he murdered 15-year-old Mary.
"A member of my team has spoken to Steve West and we will consider all options and line of inquiry once we know exactly what we are dealing with within the basement," said Mr Turner.
"If Fred were alive, undoubtedly he would be interviewed around the disappearance of Mary," he added.
West, who died in prison in 1995 aged 53, was previously suspected over Mary's disappearance.
He and Rose West tortured, raped and murdered an unknown number of women over a 20-year period.
Rose West was convicted of 10 murders in November 1995 and is serving a full life tariff.
Mr Turner said Mary’s family were being kept up to date with developments.
He said: "Clearly since 1968 the family haven't known what's happened to Mary. There has been some speculation that she may have been linked to Fred West at that time and there was conflicting information within the investigation around that.
"But it is really important to them that we do actually now search for the truth and find out once and for all if Mary is buried under the basement."
In a statement, Mary's family said: "We are extremely happy Gloucestershire Police is continuing to try and search for Mary and this gives us a chance to potentially put her at rest after all these years. We hope this is a chance to finally get closure for Mary.”