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Steve Fulcher caught serial killer Christopher Halliwell then resigned from the force as his methods came under question.

Steve, 52, was investigating the disappearance of Sian O’Callaghan, 22, who went missing in Swindon after leaving a nightclub in 2011.

The former Detective Superintendent got a confession from Halliwell not just for the murder of Sian, but a second as well.

Her stabbed body was found five days later and Halliwell admitted murdering her.

However, the manner in which Steve gained Halliwell’s confessions is the subject of tonight's To Catch A Serial Killer - With Trevor McDonald.

(Image: ITV Picture Publicity) (Image: PA)

The one-off documentary provides a vivid insight into Halliwell’s horrific crimes and explores whether Fulcher's actions were a brilliant piece of detective work, or the result of his own 'arrogance and recklessness'.

Fulcher explains how he formed a bond with taxi driver Halliwell after arresting him over the disappearance of 22-year-old Sian O’Callaghan.

The killer confessed to her murder, then revealed he had also killed prostitute Becky Godden years earlier and left her body at another location.

He was found guilty of murdering Becky, who vanished in 2003, and sentenced to a whole life term.

(Image: Wiltshire Police/PA)

The manner in which Steve gained Halliwell’s confessions breached police procedure by failing to caution him and denying access to a solicitor.

This almost derailed the court case against Halliwell and called into question one of the most important principles of the criminal justice system.

Fulcher says: "My view is that I have brought two daughters back to their mothers and I’ve prevented other victims resulting from Halliwell’s continued pursuit of his career as a serial killer.

"But for my course of action Becky would still be in that field, Sian would never be found and Christopher Halliwell would be walking the streets."

(Image: PA)

Steve was hailed by his superiors but Halliwell refused to comment any further in police interview, compromising those confessions.

Once the case reached court the judge ruled the confessions to both killings were inadmissible as evidence.

The police reopened the case and investigators discovered soil on a spade which placed Halliwell in the field where Becky was found, leading to his conviction by jury in 2016.

Steve had already resigned from the force after a series of warnings, but he has no regrets.

(Image: PA)

Sian’s mother, Elaine Pickford, says what he did was understandable to her family.

Elaine says: “Steve did what I think any family member would want an officer to do for them.”

But criminal defence barrister Sebastian Gardiner says their must be a way to ensure "police powers are not abused".

He says: “It wasn't ignorance on his part, it was a conscious decision by him to drive a coach and horses through recognised police procedure.”

*To Catch A Serial Killer - With Trevor McDonald airs tonight on ITV at 9pm