Two double murders that took place in the mid to late 1980s shocked the locals in Pembrokeshire, Wales.
It took more than 20 years before the cold case was finally reopened and solved – and John Cooper convicted of his crimes.
The evil serial killer’s first victims were brother and sister Helen and Richard Thomas, who were shot at their manor hour near Milford Haven in 1985 – before their home was set on fire.
Four years later Cooper murdered husband and wife Peter and Gwenda Dixon on a coastal path in Pembrokeshire, on the last day of their holiday.
The investigation into their deaths is dramatised in ITV's new true crime drama The Pembrokeshire Murders.
Detective Superintendent Steve Wilkins decided to reopen the murder investigation in 2006 – when forensic science was much more advanced than it was in the 1980s.
The serious crime unit at the Dyfed-Powys police force were convinced Cooper, who was at the time serving a 14 year sentence for a string of violent burglaries, was behind the murders too.
But they lacked the evidence they needed to convict him for the gruesome murders.
After years of searching, it was Cooper’s own wife who unknowingly provided key evidence that helped catch him.
In a police sketch from 1989, the killer could be seen using victim Peter’s card at an ATM after the murder.
In the sketch the killer could be seen wearing a pair of shorts, which were similar to some khaki shorts seized from Cooper.
The shorts turned out to be a trophy kept by Cooper from victim Gwenda – and his wife Pat had turned them up for him.
In doing so she had trapped vital DNA evidence in the seams of the shorts.
Mr Wilkins said: “We found the DNA of Peter Dixon’s daughter trapped in the seam."
He continued: "Now she’d been in Cyprus at the time of the murders so how the hell does her DNA make it into shorts owned by John Cooper unless they actually belonged to the Dixons?
“It’s absolutely incredible to think Pat’s simple act would lead to him finally facing justice.”
It also helped tie Cooper to an attack on five teens in Pembrokeshire in 1996, where two of the girls were subjected to serious sexual assaults.
Cooper’s appearance on the popular TV game show Bullseye also helped convict him.
A still from the episode he appeared in was used to compare against the sketch which was drawn at the time.