“Blue material” thought to be evidence that missing teenager Mary Bastholm – long suspected of being a victim of serial killer Fred West – was buried beneath a cafe may have been from a documentary crew drilling into a pipe, police have said.

Excavation work began earlier this month after police received information from a production company making a documentary that human remains could be buried in the cellar of the Clean Plate cafe in Southgate Street, Gloucester.

Mary, 15, was wearing a blue dress and blue coat and had a blue bag with her when she went missing in 1968, at a time when builder West was abducting girls in Gloucester.

But on Thursday, Gloucestershire Constabulary said teams of experts had failed to find any human remains, and that the “blue” material was likely to be connected to a buried pipe.

Assistant Chief Constable Craig Holden told reporters outside his force’s headquarters in Quedgeley: “When we were called by the production company it was because they had seen an image of what appeared to be blue material.

“When our senior investigating officer and our experts looked at the footage from the endoscope that had been deployed by the TV production company it was clear there was imagery of something that was blue and it couldn’t be identified to what that was.

“Under excavation we identified a pipe that wasn’t blue.

“And we believe that the drilling may have gone through that pipe and created some form of detritus or debris which is what the endoscope has picked up.”

He said the current cost of the excavation work to taxpayers was around £50,000 with the potential to rise to £60,000 to £70,000.

The potential amounts did not include the reparation costs of discovered items or structures which may be “of archaeological significance”.

He also said there was no “legal basis” to seek costs from the production company which tipped police off, but added: “Although, as a gesture of goodwill, if they were prepared to provide something then that’d be something we’d need to discuss with them and, of course, the cafe owners.”

Mr Holden said areas of interest had been “methodically and meticulously investigated” but “no human remains of any relevance to our inquiry were found”.

He said the force would review the activity of the production company as well as the force’s engagement with it and its response.

He added: “Our thoughts are with Mary’s family, whose continued dignity over recent week and over many years before has been absolutely humbling.

“I am now confident that Mary is not buried in the cellar of the cafe, and I hope that eliminating this location will bring some small comfort to her family.”

Mary’s surviving family said in a statement they are “still very sad” that her body was not recovered, but continue to “hold out some hope that one day Mary will be found”.

“We were hoping to get final closure on her disappearance so that we could put her to rest,” they said.

“We have been open-minded throughout this process and we now know the cafe can be ruled out.

“We can however hold out some hope that one day Mary will be found.”

A spokesman for The Nelson Trust, which runs the Clean Plate cafe, said: “As we said at the outset, our support of the police was always driven by what we thought were in the best interests of the family.

“They remain in our thoughts and prayers.”

Mary worked as a waitress at the Pop-In cafe, which was located on the same site as the Clean Plate.

Serial killer West, who died in prison in 1995 while awaiting trial accused of 12 murders of women and girls, knew Mary and was a known customer of the cafe.

Officers were previously seen carrying objects including a number of wooden doors out of the cafe while forensic archaeologists said there were a number of structural anomalies in the cellar including six voids in the floor that were being examined.

Fred and wife Rose West, 67, tortured, raped and murdered an unknown number of women over a 20-year period.

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

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.

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 a life sentence.