Serial killer Fred West would be interviewed by police investigating the disappearance of a 15-year-old schoolgirl in 1968 if he was still alive, a senior detective has said.

Police are due to begin excavating the cellar of a cafe in Gloucester after finding “possible evidence” suggesting the body Mary Bastholm might be buried there.

Forensic archaeologists have been undertaking exploratory work at the Clean Plate cafe in Southgate Street in connection with the disappearance of the teenager who was last seen alive in January 1968.

Gloucestershire Police had received possible evidence from a TV production company to suggest the body of the missing girl could be buried at the location.

One of the findings presented to police was a photo taken by the production company of what appeared to be blue material buried in one area of the cellar. Mary was wearing a blue dress and blue coat and had a blue bag with her when she went missing.

Forensic archaeologists have confirmed that there are a number of structural anomalies within the cellar that warrant further exploration and six voids in the floor will be examined.

Detective Chief Inspector John Turner, who is leading the investigation, said West was a regular at the cafe – then called the Pop-In – and knew Mary as she worked there as a waitress.

“Fred West was always indicated as being involved in Mary’s disappearance and possibly her death,” Mr Turner said.

“We will be searching the basement to find out what, if anything, is there. If anything is found we will carry out a forensic review and we will look to seek people within the investigation that we need to interview.

“If Fred were alive, undoubtedly he would be interviewed around the disappearance of Mary.”

On Tuesday, police were seen carrying objects including a number of wooden doors out of the cafe. The force has confirmed that excavation work will not begin until at least Wednesday.

The TV production company had identified a void within the floor underneath the toilet in the basement and a cadaver dog indicated there may be something of interest within the cavity.

Serial killer West, who died in prison in 1995 aged 53, was previously suspected over Mary’s disappearance.

“He is the only one who really knows if he had involvement in Mary’s death,” Mr Turner said.

When Mary disappeared, West was living with his first wife Rena at a caravan site at Bishop’s Cleeve, near Cheltenham – close to where a teenage Rose West was living with her parents.

Mr Turner said his “understanding” was that Mary’s disappearance in January 1968 happened before Fred had met Rose – but he did not rule out speaking to her in prison.

“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,” he said.

The Wests tortured, raped and murdered an unknown number of women over a 20-year period.

Mary’s disappearance in 1968 was at a time when builder West was abducting girls in Gloucester.

West is said to have confessed to killing Mary in conversation with his son, Stephen, but never admitted it to police.

Detectives have recently spoken to Stephen West, Mr Turner said.

“There is no evidence at this time to link Fred West with Mary’s disappearance although he did tell his son Steve West just before he died that he had murdered Mary,” he said.

Mary’s body was not found during the 1994 excavation of the Wests’ home, now known as the “House of Horrors”, on Cromwell Street in Gloucester.

The detective said there was conflicting evidence during the original House of Horrors inquiry as to whether West had carried out renovation work at the cafe in 1968 or later, in 1976.

“Since then there has been a lot of rumour and folklore about Mary and Fred’s link with the cafe and the decision was made that there was insufficient evidence at the time to excavate the site,” Mr Turner said.

“I met the Bastholm family on Monday and spoke with them at length about what we were going to do and the plan moving forward.

“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 and would like to continue to ask for privacy whilst the excavation is ongoing.”

West was charged with 12 murders, but took his own life in a Birmingham prison before his trial.

Rose West was convicted of 10 murders in November 1995 and is serving life.