Inside story: How a resourceful detective got Chris Halliwell to confess – but fell foul of the law

Officer solved the case but now faces inquiry into his illegal tactics. By Paul Peachey

After five nights without any sign of Sian O'Callaghan, all hope of finding her alive had rested with the abductor revealing her location. That, at least, was the conclusion of the sleep-deprived detective leading the hunt as he paced the floor at Gablecross police station in Swindon awaiting news from his officers on the front line.

Detective Superintendent Steve Fulcher had a suspect: Chris Halliwell, the owner of a taxi which was picked out by CCTV close to where the 22-year-old office worker tottered out of Suju nightclub in Swindon in the early hours of 19 March last year after a night out with friends.

Close surveillance, set up in the hope that he would lead the Wiltshire police team to where she had been hidden, had proved fruitless. So the last hope for Det Supt Fulcher was that Halliwell would talk after his arrest in a supermarket car park. When he snatched up the phone to speak to his colleagues, he learned that he had refused to comment.

Those were the facts facing Det Supt Fulcher when he took the remarkable decision to order Halliwell to be taken to an isolated hilltop where he took personal charge of his questioning.

Over four extraordinary hours, the experienced policeman and the suspect embarked on a tour of rural southwest England. Halliwell first led the detective to the site where he had dumped Ms O'Callaghan's body.

And then in a remarkable unburdening of past wrongs, he took him to a field where he had dumped another body some eight years earlier. Until then, nobody had even known that Rebecca Godden-Edwards was dead.

But the detective's unconventional tactics were "serious and irretrievable" breaches of the rules surrounding police questioning and meant that Halliwell could not stand trial for Ms Godden-Edwards murder. And it ensured that Det Supt Fulcher now faces an independent police watchdog inquiry into his actions.

By the time of the arrest, it was clear that the pressure had been growing for days, both on the police and on the man they were hunting. Halliwell had become the suspect by the time he was questioned on 23 March, four days after Ms O'Callaghan had been reported missing by her boyfriend.

Det Supt Fulcher had been happy to let him "run" watched by surveillance teams but he was forced to act when Halliwell headed to Boots to buy a big enough stash of paracetamol to kill himself. Halliwell was approached in an Asda car park as he was picking up a fare and wrestled to the ground. Following a plan, officers carried out an "urgent" interview in the police car to try to extract information from him – a tactic allowed without a solicitor present if it could save the life of Ms O'Callaghan. When it revealed nothing, police were heading to the station when Dept Supt Fulcher ordered them to divert and take Halliwell to Barbury Castle, the site of an Iron Age hill fort and a beauty spot popular with walkers and horse riders.

It was, he said later in court, a last opportunity to "look him in the eye and to ask him this one thing – will you take me to Sian?"

Accompanied only by a civilian note-taker, Det Supt Fulcher led Halliwell 50 yards from the police cars and conducted a nine-minute interview. With the murder of Joanna Yeates in Bristol still fresh in the memory, the detective warned the suspect that he would be vilified if he did not tell him where she was.

According to the notes, Halliwell first told the detective: "I don't know anything" and repeatedly asked to see a solicitor. "You think I did it," he told the detective. "I know you did it," came the response. By the end of the interview, Halliwell told the officer: "Have you got a car – we'll go."

They continued to speak in the back of a police car for 45 minutes until they reached Uffington, the site of the medieval horse etched into the hillside. Halliwell gave enough information for specialist search teams to find the body later that afternoon.

It was at this point, when Det Supt Fulcher prepared to have him charged with murder, that Halliwell told him: "We need to have a chat."

They drove a short distance, sat down together on the grass and had a cigarette. Halliwell became more candid, saying that he was a "sick f***er" and asking the detective if he wanted "another one".

They drove 30 miles into south Gloucestershire where Halliwell pointed to an area in a 40-acre field where he said he had buried another woman between 2003 and 2005. Police later found the remains of Rebecca Godden-Edwards, who had been working as a prostitute when she was killed.

The story emerged in February but can only now be made public. Halliwell's team insisted the way the evidence was gathered breached the Police and Criminal Evidence Act, which gives any suspect the right to legal advice and to be questioned without oppression. It was "an assault on the integrity of the legal system," said Ian Latham QC, for Halliwell.

When asked to justify his tactics, Det Supt Fulcher told the court: "I did think it was utterly ridiculous that someone who took me, 12 people and a surveillance helicopter to the deposition sites of two bodies would then seek to find some loophole or quirk in the law to get away from the fact that he was a multiple murderer."

He was backed by senior colleagues including the deputy chief constable, Patrick Geenty, who called it a "gutsy decision". However, the trial judge, Mrs Justice Cox, ruled that none of the evidence gathered during the four-hour period could be used in a trial. With no other evidence linking Halliwell to the murder, prosecutors were forced to drop the charge.