Detectives have revealed how footage of a violent killer's appearance on the TV game show Bullseye helped them to finally put him behind bars, after he evaded capture for decades.

Dubbed the 'gameshow serial killer' John Cooper, now 73, from Pembrokeshire in Wales, appeared on the darts quiz show in 1989 - four years after he'd murdered reclusive siblings Richard and Helen Thomas, and just weeks before he would go on to violently execute husband and wife holidaymakers Peter and Gwenda Dixon.

Despite being suspected of the murders, Cooper, who was known to police as a prolific burglar, spent years at large due to a lack of evidence linking him to the crimes.

It was only when cold case detectives got hold of the archive footage from the Jim Bowen fronted show in 2009 that they had a breakthrough - after managing to freeze the frame on Cooper standing in the exact same position as an artist's impression of the wanted killer sketched at the time of the original investigation.

The similarity was such that it looked 'like a tracing', said lead detective Detective Chief Superintendent Steve Wilkins, in the ITV show The Gameshow Serial Killer Police Tapes, which recounted the extraordinary tale.

Advances in DNA and scientific evidence meant that, when police finally questioned Cooper, they were able to link a gun used in a robbery he had been convicted of to the murder weapon used to kill the Dixons.

When cold case detectives got hold of archive footage of John Cooper's 1989 appearance on the darts quiz show (seen left) in 2009, they were able to freeze frame Cooper in the exact same position as the man in an artist's impression of the killer sketched at the time of the crime. The striking resemblance was enough for them to pursue an investigation into Cooper again, and advances in forensics meant they were able to link a gun used in a robbery he was convicted of to the weapon used in the then unsolved murders of Peter and Gwenda Dixon

John Cooper, pictured in 2011 at Swansea Crown Court where he was charged with two double murders of Peter and Gwenda Dixon and Richard and Helen Thomas in 1989

Reclusive brother and sister Richard and Helen Thomas were killed after Cooper shot them and then burned down their house in Scoveston Park on December 22, 1985

Cooper tied-up and robbed holidaymakers Peter and Gwenda Dixon, from Oxfordshire, after confronting them on the Pembrokeshire coastal path near Little Haven, he executed them using a sawn-off shotgun in the horrific double murder

Cooper was finally convicted of the murders in 2011 - more than 20 years after the killings.

Susanna Reid interviewed the leading detectives in the case - some of whom spoke for the first time - who revealed how thief Cooper was known to police, but had never actually been questioned by officers about the murders at the time.

Four years after murdering his first victims in 1985, Cooper had appeared on Bullseye.

Just weeks later, he shot Peter and Gwenda Dixon as they enjoyed a coastal walk on the final day of their Welsh summer holiday.

The diagnosed psychopath had appeared on gameshow Bullseye just weeks before the vicious shooting of a couple on a holiday in Wales

Detectives had suspected Cooper of the killings since the early 1990s after noticing similarities between his modus operandi as a burglar, and that of the man behind the unsolved murders.

When cold case officers revisited the crimes, Cooper was in prison for a violent robbery, and approaching the end of a 16 year sentence.

The detectives brought him in for interview in the hope of wrestling a confession from the criminal, but initally made no ground.

Susanna Reid spoke with detectives in the cold case team about how they finally captured the killer after 20 years

Cooper was brought in for interviews by the team but he denied any guilt insisting that he had not murdered anyone

Lead detective Det Chf Supt Steve Wilkins said: 'What we managed to do then was freeze John Cooper in exactly the same position as the artist’s impression, and for me, it was like a tracing.'

Along with the striking similarity to the artist's sketch, Cooper discussing the area of Pembrokeshire also led detectives to believe he was their killer.

On the episode of comic Jim Bowen’s darts quiz, the killer chatted with host Bowen about his 'unusual hobby'.

'Oh yes... The scuba diving,' a charming Cooper said.

Bowen remarked Pembrokeshire was 'the place to do it down there, isn't it?'

Cooper responded: 'Oh we’ve got the coastline yes... Well we’ve got deep water where you can sort of swim over mountains, and all sorts.'

While the interviewers tried to catch Cooper in a lie, it was clear they needed forensic evidence against him, specifically to link him to a gun found near his home after his arrest for burglary.

After serving time in prison for robbery he was then convicted at trial of the double murders and sentenced in 2011

FORENSIC EVIDENCE STRENGTHENS THE CASE

But Cooper was forensically aware, says forensics lead Glan Thomas, which hampered the chances of getting any evidence off the gun even with advances in science.

He says: 'He was no fool, he’d go gloved up, he’d go masked up. There were no fingerprints, there was no DNA evidence. So basically, I am looking for that golden nugget.'

But when no breakthrough came, Cooper was released back to his home. His wife died shortly afterwards through natural causes.

In despair, lead detective Steve called in head of forensics Dr Angela Gallop, who he instructed to undertake a new round of fine fibre testing on Cooper’s clothes from the murder scenes.

She says: 'We noticed a tiny fragment of what looked like blood. We did some DNA profiling, then I rang Steve Wilkins ‘cause I thought this is probably what he was after.

'We got a blood flake from the shorts from Cooper’s home that matches Peter Dixon’s.'

Steve says: 'I just was banging the steering wheel, I was shouting, a few tears. It was a real emotional moment... Put the picture of the shorts up, with the words, "Got the b******!"'

Cooper was tried at Swansea Crown Court and after an eight-week trial the serial killer was also convicted of separate offences of rape, sexual assault, and attempted robbery - he will never be released

EVADING CAPTURE FOR 20 YEARS

Cooper had a lengthy criminal history, including 30 robberies and a violent assault - for which he was arrested and sentenced to 14 years in 1998 for burglary and robbery.

A diagnosed psychopath, Cooper - who was married to late wife Patricia and had two children - was a prolific thief before he turned killer.

Reclusive brother and sister Richard and Helen Thomas were found shot dead in the burned out wreckage of their farmhouse.

Then four years later, in broad daylight on the popular Welsh coastal path, when 51-year-old Peter Dixon and his 52-year-old wife Gwenda were found shot dead.

Despite a huge manhunt, no-one was caught for either of the double murders, and the trail ran cold.

Det Insp Louise Harries explained to Susanna why the team reopened the case 20 years later.

'We obviously wanted a result because you had horrendous offences that had happened, and there were the victims families out there, who were still all these years later looking for answers, so you have that feeling of burden that you want to get justice for those families.'

The Gameshow Killer: Police Tapes airs on ITV tonight at 9pm