He has pleaded guilty to one murder, he told police of a second. But has killer Chris Halliwell taken the lives of even more young women?
When Halliwell, now 48, was arrested in March last year, neighbours spoke kindly of the man who lived in a semi in a suburban part of Swindon.
Brian Jerome who lived nearby said at the time: 'He's a really nice guy. He'd always have a smile and he'd often be out working on his car.
'I couldn't believe it when I heard. Nice man, nice family, nice girls.'
Halliwell lived with his partner Heather Widdowson and her three daughters. He was a father to three children himself with his ex-wife Lisa. Friends spoke of a devoted dad and family man.
Others say he was the kind of person who would come out in the middle of the night to help them.
Neil Barnett is a minicab driver who has known Halliwell for four years.
– Neil Barnett, Minicab driver
Real nice bloke, a genuine bloke, a normal run-of-the-mill bloke. I've got two daughters I would have trusted them in his car. When I found out - phew. You change your view of people.
Yes, this respectable, kindly, helpful man had a darker side, a side which none of his friends, children, stepchildren or neighbours knew about.
Halliwell was a regular customer of prostitutes. After his arrest, police questioned women working Swindon’s streets. Some said they had been picked up by Halliwell in the weeks preceding Sian’s death. They said he got rough, weird with them. Some got frightened and called for help. But Halliwell backed down - on those occasions.
Sian O’Callaghan was not so lucky. The 22 year old personal assistant was in the wrong place at the wrong time, leaving the Suju nightclub just at the moment as he was driving past.
And when he was arrested he told detectives of a previous killing. He could not remember when he killed this historic victim, he didn’t know her name. He just knew he had killed her and left her body buried in a field at Eastleach near Cirencester.
The woman proved to be Becky Godden. But were there others? Why would he kill a woman in 2003 and then wait nearly a decade to take another life? What prompted him to kill at those times and not others?
Police quickly sought to see if there were other, outstanding missing women cases which he could be linked to. They also ran checks on his background.
The picture that emerged was of a man who had a brutalised boyhood, abandoned by uncaring parents. Police sources say he clearly had ‘issues’ with women.
Sources close to the investigation say one potential line of inquiry links Halliwell with the disappearance of Sallyann John. The 24-year-old prostitute disappeared from Swindon in September 1995. Although her family believed she was moving away to start a new life in London, she has not been seen since. At the time of her disappearance, Halliwell was living along the same street in Swindon as Sallyann.
As for other potential victims, there was press speculation linking Halliwell with the death of Melanie Hall in Bath in 1996. Her disappearance was similar to that of Sian’s, vanishing after she left a nightclub, her body eventually found in 2009, abandoned by the M5. But police are still open-minded about this connection and say it deserves further investigation.
Before Halliwell was arrested, he reportedly told a colleague at his minicab firm: “Who knows what or who you will find buried out there, there could be loads of people over the years.”
The fact is only one person knows exactly how many women he has killed, and that is Halliwell himself.