Police are searching for a body at a cafe in Gloucester linked to a girl feared to have been murdered by serial killer Fred West.
Mary Bastholm, 15, went missing on 6 January 1968 and has not been found.
Officers were called to The Clean Plate by filmmakers working on a documentary. West was a regular customer when the teenager worked at the cafe.
The film crew told police on Friday they had found evidence that a body could be buried within the property.
Officers could be at the site "for a number of weeks" and excavation work is yet to begin, Gloucestershire Police said.
Assistant Chief Constable Craig Holden said: "This could be a significant development on a case which has gained a lot of public interest over the years.
"On the basis of the information provided to us, we consider this to be new and potentially important evidence in the case of Mary Bastholm, however further detailed assessments will take place over the following few days to determine the extent of excavation needed.
"First and foremost our thoughts are with Mary's family and we would ask that their privacy is respected at this difficult time."
In a statement released via the force, Miss Bastholm's family said: "We are aware of the ongoing developments around Mary's disappearance and are being kept up to date and supported by Gloucestershire Police family liaison officers.
"At this time please may we ask for privacy so we can grieve for Mary."
A white tent has been set up outside the building in Southgate Street and officers are stationed at the scene.
The cafe is close to Cromwell Street, where officers found the bodies of West's victims at the home he shared with his wife Rosemary.
A spokesperson for The Nelson Trust, a women's charity that owns The Clean Plate, said: "As soon as we were made aware of the situation, we have done everything we can to accommodate the investigation.
"The Clean Plate will remain closed until all necessary activity is complete. "It is an extraordinary twist of fate that this building, now associated with a story as tragic as that of the victims of Fred and Rosemary West, is part of a charity working to support women at risk from violence."
At the scene
By Andrew Plant, BBC News
Mary Bastholm disappeared at a bus stop on her way to meet her boyfriend.
There has been no trace of her since. It's been a mystery for 53 years.
The cafe has been there for decades and has been under several different names.
One of its waitresses was Mary, who was 15 at the time when Fred West was a regular customer and was known to him.
It was also known he was a builder at the time renovation was needed on the cellar and the toilets at the cafe - work he was believed to have carried out in 1968.
A lot of people locally put two and two together and suggested that perhaps he was responsible for her disappearance and that was where she ended up.
Police say their activity will take place over a number of weeks and there will be disruption for some time.
In 2012, a petition was set up calling for a fresh investigation into Miss Bastholm's disappearance but it was rejected by then Chief Constable Tony Melville.
In 1994, West admitted murdering his daughter Heather.
The confession came as police began to search 25 Cromwell Street, where they found the bodies of nine girls and young women.
He and his wife Rose were jointly charged with nine murders and he was charged with a further three.
West took his own life while awaiting the murder trial the following year.