Witney victims the Dixons on ITV’s The Pembrokeshire Murders

The Pembrokeshire Murders, which aired on ITV last week, told the story of murderer John Cooper and how the reopening of the investigation years down the line brought him to justice.

In June 1989, Witney residents Peter and Gwenda Dixon were on a camping holiday in Little Haven, Pembrokeshire.

After being reported missing by their son, their bodies were found six days later.

From left to right: Peter Dixon, Gwenda Dixon, Richard Thomas and Helen Thomas. Picture: ITV

The evidence of the Home Office pathologist, Bernard Knight, confirmed that Mr Dixon was shot three times, with his hands tied behind him, while Mrs Dixon was shot twice in an execution-style killing.

They were killed with a sawn-off shotgun fired at close range, while Mr Dixon’s NatWest cash card and wedding ring were missing.

Inquiries revealed that Mr Dixon’s card was used four times after his death and witnesses helped create an artist’s impression of a man seen using the card to withdraw cash.

A pair of khaki shorts in the artist’s drawing were seen as crucial to the case.

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Meanwhile, in December 1998, Cooper was jailed for 16 years, after being found guilty of 30 burglaries and one robbery.

From 2006 to 2011, Operation Ottawa saw the investigation into the death of the Dixons and two other murder victims reopen.

The senior investigating officer, Steve Wilkins, used a televised public appeal on ITV Wales Tonight to say scientific advancements and DNA were making it possible to catch the culprit.

The Pembrokeshire Murders was based on the real life crimes of John Cooper. Picture: ITV

When the khaki shorts were finally discovered, a fleck of Mr Dixon’s blood was found on them.

Meanwhile, three weeks before the Dixons were murdered, Cooper appeared on TV game show Bullseye, allowing the police to compare the artist’s drawing with a freeze-frame of Cooper, revealing a startling resemblance.

At Cooper’s trial, he was found guilty in May 2011 of four murders (including the Dixons), a rape and a sexual assault.

Serial killer John Cooper. Picture: ITV

On the accompanying ITV documentary, Catching the Gameshow Killer, Mr Wilkins said of Cooper: “He is the most dangerous human being that I’ve ever come into contact with.

“From day one, I felt we would find the person responsible. Evidence is always there, you’ve just got to find it.”

Operation Ottawa detective Steve Wilkins. Picture: ITV

In 1985, Cooper shot dead millionaire Richard Thomas and his sister Helen, after being caught breaking into their home, while in 1996, Cooper held a group of teenagers at gunpoint as they played in a field.

He raped one and sexually assaulted another.

Cooper is currently serving four whole-life sentences.

The ITV crime thriller is based on a book written by Mr Wilkins and ITV journalist Jonathan Hill, who followed the story closely.

The duo formed a strong friendship, and the book, The Pembrokeshire Murders: Catching the Bullseye Killer, was published shortly after Cooper was imprisoned.