DETECTIVES are looking into the murky past of killer cabbie Chris Halliwell with the theory there may be more victims – even beyond Swindon.

The 48-year-old was handed a life sentence on Friday after admitting killing Sian O’Callaghan in March 2011, but work has continued for police investigators, who will be analysing the father-of-three’s movements over the last 30 years.

It comes as one national newspaper reported how, when he was serving a jail term for burglary in the 1980s, he asked a cellmate ‘how many do you need to be a serial killer?’ A police source said the Wilt-shire force would be working closely with serious crime experts at the National Police Improve-ment Agency, where former lead investigator of Operation Mayan Det Supt Steve Fulcher served as an adviser until August.

“There will be extensive research into his lifestyle and background and we will start to cross-reference his movements against other unsolved crime,” the source confirmed.

“It is not a two-minute job and is likely to go on for months if not years.

“It will be a large-scale analytical exercise and certainly it is early days but this potentially goes much further than Swindon.

“We cannot say what else there might be – it would be like putting a finger in the air and guessing.

“We have to apply some science to it and most of the activity will be around intelligence, research and analysis.

“We will engage with the NPIA, who have details of all unsolved serious crimes in the country.”

The second charge of the murder of Becky Godden-Edwards was dropped even after Halliwell had led officers to within a few feet of her remains, as a judge ruled the lead detective Steve Fulcher had breached the Police and Criminal Evidence Act when interviewing the killer.

Keith Vaz MP, chairman of the Commons home affairs select committee, said it could be time to reform the Police and Criminal Evidence Act.

He said: “We should never be in a position where regulations prevent people being brought to justice.”

“The Act was fashioned almost 30 years ago and policing has changed enormously since then. “Of course we need regulations and rules concerning how people are interviewed and also we need to recognise that policing has changed.”