A diagnosed psychopath, Cooper – who was married to late wife Patricia and had two children – was a prolific thief before he turned killer.
Richard and Helen Thomas were found murdered at their home, Scoveston Manor, near Milford Haven, in December 1985. They had been killed in an ‘execution style’, with both suffering shotgun wounds.
Police speculated that there may have been an altercation between the two but soon decided a third party was involved. Cooper sprinkled paraffin around the house and set it alight.
It was speculated that Cooper, a local in the area, was jealous of the millionaire farmers and targeted them at the secluded stately home.
He was known to enjoy the Pembrokeshire coast – even mentioning it on Bullseye – where he intercepted the walk of his second set of victims.
Cooper shot Peter and Gwenda Dixon in the face with a sawn-off shotgun as they enjoyed a coastal walk on the final day of their Welsh summer holiday on the Pembrokeshire coast, in June 1989.
He hid their bodies in nearby bushes. Cooper also stole money from them, using their bank cards to withdraw £300.
Cooper was sentenced to fourteen years in 1998 for robbery and burglary, enabling the police to collect further evidence against him.
While going over the material from the investigations, Steve hired criminal psychologist Dr Adrian West to build up a profile of the killer.
He told The Sun: ‘I asked Adrian, “How dangerous is this guy?” and he said, “Steve, I have only come across two people in my professional life who if I found in my bedroom in the middle of the night, I know I would have to kill them to survive. One is Donald Neilson, the so-called Black Panther, and the other is John Cooper.” And I just went cold. I just thought, yes, this is a dangerous guy.’
Cooper (pictured in 2009 as he was led into court) was sentenced to fourteen years in 1998 for robbery and burglary, enabling the police to collect further evidence against him
Cooper was brought in for interviews but denied any guilt, insisting he had not murdered anyone
Pictured: the gun John Cooper used to murder the Dixons and items of clothing found at the time
Steve kept the reopened investigation, codenamed Operation Ottawa, secret before revealing live on ITV Wales News in November 2007 that he was carrying out a cold-case review and made a renewed public appeal for witnesses.
A breakthrough came in 2009 when archive footage from Bullseye, fronted by Jim Bowen, was obtained and officers managed to freeze a frame of Cooper standing in the exact same position as an artist’s impression of the wanted killer sketched at the time of the original investigation.
Steve said the similarity was such that it looked ‘like a tracing’. Along with the striking similarity to the artist’s sketch, Cooper discussing the area of Pembrokeshire on the show also led detectives to believe he was their killer.
Having noticed the shorts recovered from Cooper’s home were shorter than the artist’s impression, Steve asked forensic experts to unpick the hem – which is when they found the traces of Mr Dixon’s DNA.
Father-of-two Steve, now 61 and living in Cheshire with his second wife Diane, 56, grew up in Pembrokeshire and is played by Welsh-born Hollywood star Luke Evans in the series
He explained: ‘What had happened was Cooper’s wife, who was a seamstress, had shortened the shorts and in doing so she had actually sealed in some of the forensic evidence.’
Cooper had been released from prison on parole in January 2009, but was arrested again four months later for the double murders, a serious sexual assault and five attempted robberies in Milford Haven in 1996.
Steve told how when he was arrested, he was found with a map, gloves and a rope in the boot of his car.
Although Cooper denies his guilt, he was convicted of two double murders and jailed for life in May 2011.