The hard and uncomfortable lessons from the investigation into Sian O'Callaghan's murder

So what is moral is not always legal.

That's the hard and uncomfortable lesson of the investigation into the disappearance of Sian O'Callaghan.

The detective in charge backed his instincts, caught the killer and uncovered a second murder.

But he is now suspended and under investigation for breaching the rights of the man he persuaded to lead him to two bodies.

That man was Christopher Halliwell who pleaded guilty today to murdering 22-year-old Sian after she vanished from a Swindon nightclub last year.

He was a suspect within hours because his car was seen on CCTV and its number plate was recognised by automatic cameras.

Hoping he would lead them to Sian, or at least her body, police tailed Halliwell for hours but there was no breakthrough.

Det Supt Steve Fulcher then used the media to apply pressure announcing he expected the case to be solved in 24 hours.

Halliwell then headed to a chemist and bought enough pills to kill himself.

Police moved in to prevent a suicide attempt and began to question him in the back of their car - without a lawyer.

Fulcher believed there was chance Sian could still be alive and that a "safety interview" designed to elicit potentially life-saving information was justified.

But Halliwell would not talk so Mr Fulcher upped the ante.

He arranged for his suspect to be driven an Iron Age fort near where he believed Sian might be.

There, he met Halliwell himself and offered him a deal: take me to Sian and I'll rehouse your family when the press get wind of your arrest.

Halliwell replied: "Have you got a car? We'll go."

Mr Fulcher's tactic had worked and Halliwell led him to Sian's body in a ditch near the Uffington White Horse.

She had been partially stripped and stabbed in the back of the head and neck. Police now believe Sian suffered terribly from the moment she was snatched.

Halliwell asked to be taken away and the group parked up the hill near the chalk monument.

Handcuffed, he went for a walk with Det Supt Fulcher.

"We need to have a chat…", he told the detective.

Then he asked: "Would you like another one?"

Mr Fulcher felt unable to refuse the offer of another body, a split-second decision that would later be picked over for days.

Halliwell led officers to a field 17 miles away at Eastleach.

En route he remarked: "I'm a sick f* … Normal people don't go around killing each other."

From a point in the dry stone wall where some rocks had slipped Halliwell paced out a distance and stopped.

Mr Fulcher placed an officer on the spot and a day after the diggers moved in the remains of another woman were found.

Halliwell did not know her name nor the year she was abducted but police discovered she was Becky Godden-Edwards, a troubled young woman who had lost touch with her family in 2003.

It seemed Fulcher had uncovered a potential serial killer.

But there was a problem.

When Halliwell was taken to a police station for full interview and given a lawyer he chose not to repeat his admissions about Sian or Becky.

Months later his lawyers argued the "safety interview" should have been stopped before he met Fulcher.

Halliwell's barrister recognised there had been a "moral purpose" to the police tactics but insisted they had been unlawful.

At Bristol Crown Court last year Mrs Justice Cox agreed and ruled his confession to both Sian and Becky's murders were the result of "oppression."

As such, everything Halliwell said and did after meeting Fulcher was inadmissible as evidence.

Forensics and CCTV meant police could salvage the case against him for Sian's murder.

But for Becky's family there would be no justice.

The breaches of Halliwell's rights meant he could not be prosecuted for her murder - even though he had led police to her shallow grave.

For Becky's family there are conflicting emotions.

Her mother Karen is angry not at the police but the courts. Fulcher is, she says, a good man who found out what happened to her daughter and allowed her to receive a decent burial.

Becky's father John takes the opposite view and has complained to Wiltshire Police about the officer's conduct.

The Independent Police Complaints Commission is currently investigating and DSi Fulcher has been suspended over an unrelated matter.

Halliwell will serve life, unlikely to ever be released.

He seems to have got away with one murder - only he knows if there may be even more.