It was 1989 and police detectives in Pembrokeshire in southwest Wales suspected they had a serial killer on their hands.

Holidaymakers Peter and Gwenda Dixon had been shot dead while out for a walk together on a costal path. Peter Dixon had been robbed, and his wife sexually assaulted, as reported by Wales Online.

The crime bore a striking resemblance to another committed four years earlier.

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Richard Thomas and his sister Helen Thomas had been found shot dead in the burned out wreckage of their farmhouse, as reported by The Guardian.

Officers felt sure the murders were linked – the work of a serial killer – but despite a large-scale manhunt, no arrests were made.

Then, in 2009, a cold case team picked up the file.

Speaking in an ITV documentary, The Gameshow Serial Killer: Police Tapes, Detective Inspector Louise Harries said: “We obviously wanted a result because you had horrendous offences that had happened, and there were the victim’s 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.”

Quickly, the officers noticed the similarities between the killings and the modus operandi of a local burglar by the name of John Cooper. There was even an artist’s impression of the alleged killer in the file that looked rather like him.

They brought Cooper in for an interview, but got nowhere.

Then, they had a breakthrough.

They discovered Cooper had appeared on an episode of the popular TV gameshow Bullseye back in 1989, exactly a month before the Dixon murders. They figured that by watching the episode, they would have a much clearer idea of what Cooper looked like at the time of the killings and could see if it matched up with the artist’s impression.

Detective Chief Superintendent Steve Wilkins told the documentary: “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.”

A freeze-frame of John Cooper on Bullseye alongside the police artist's impression. Credit: ITV

Cooper was already behind bars at the time, serving 16 years for a violent robbery. Attempts to illicit a confession from him were unsuccessful.

The cold case team knew they would also need forensic evidence linking him to the crime. But that wouldn't be easy because Cooper was 'forensically aware' due to his experience as a burglar.

As Forensics lead Glan Thomas explained in the documentary: "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."

And incredibly, they found one. After carrying out further forensic testing on some of Cooper's clothing, the detectives discovered a fleck of blood on a pair of shorts that matched Peter Dixon's.

"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*****d!" Steve Wilkins told the documentary.

Cooper was charged with both double murders. He was eventually convicted in 2011 and sentenced to life.