Excavation work has started at the Clean Plate Café in Gloucester in the search for evidence linked to serial killer Fred West.
Officers hope to find clues to help identify the burial location of Mary Bastholm under the café where she used to work.
Detective Chief Inspector John Turner, Senior Investigating Officer at Gloucestershire Police, said officers could find Mary under the building as well as other human remains.
He said: "I honestly don't know what we are going to find in the basement.
"It could be we find Mary, it could be we find nothing, or it could be - because the basement is on a site of archaeological interest - because Gloucester has been there since the Roman times, we may find other human remains.
"The most important thing is, I've now got the opportunity to find out once and for all what is in the basement.
"The police's job is to find the truth and the truth will be found for the benefit of Mary and for the benefit of the family.
"I think it's really important once and for all to find out what's going on and I hope this can bring some kind of closure for the family and the wider community."
Detectives announced they have unearthed significant evidence at the Clean Plate café after finding six "void" areas in the cellar where a blue material was located.
Fred West used to go to the café regularly 50 years ago, when it was known as the 'Pop-In' cafe, and Mary was employed there before she disappeared at the age of 15 in 1968.
After an ITV Trevor McDonald-fronted documentary crew presented Gloucestershire Police with new findings, a fresh search of the café began.
Both of Mary’s parents, Christian and Doreen, died before they could find out the truth about their daughter and her brother Peter passed away last summer.
It's been reported Fred told his son Stephen he had killed Mary but never admitted it to police.
Speaking about this admission, DCI Turner said: "The trouble with that is, Fred often lied to the investigating officers throughout the investigation.
"So just because Fred said it, it doesn't mean to say it's true."
Fred took his own life in prison in January 1995 while his wife Rosemary West, 67, is serving a life term in prison for her part in 10 murders.
The couple committed at least 12 murders between 1967 and 1987, torturing them in a 'sex dungeon' and burying them in their cellar and garden.
Mary was on her way to play Monopoly with her boyfriend Tim Merrett in Hardwicke when she went missing.
Tim, now 72, said he has no doubt Fred killed Mary but is unsure about whether she buried under the cafe.
He said: "To me it doesn’t make sense that she is there but I would urge the police to keep looking for her.
"I’ve never forgotten Mary, she was my girlfriend for about six months when she just disappeared.
"She was coming up to her 16th birthday. When Fred West was caught it was obvious to me that Mary was one of his victims.
"He didn’t give up the names of everyone he killed and I believe Mary was one of those."