In 1985 and 1989 two double murders shook Pembrokeshire and Wales while stumping police officers.
Police were unable to charge anyone for the brutal killings and the cases remain unsolved until a renewed investigation in 2006, when both cases were reopened more than two decades after the first victims lost their lives.
A combination of extraordinary police work, new forensic technology and an appearance on ITV's gameshow Bullseye eventually led to the killer's capture.
The unusual story is told in a new ITV drama starring Luke Evans, called The Pembrokeshire Murders.
But what actually happened? This is the true story of the Pembrokeshire murders.
This article will contain spoilers to ITV's three-part drama, The Pembrokeshire Murders, airing between January 11 and January 13.
The first murders
Brother and sister, Richard and Helen Thomas were murdered in their home in Milford Haven on December 22 1985. The killer shot the siblings and then burnt down the mansion where they lived with the bodies inside.
Police were unsuccessful in catching the perpetrator - John Cooper - who had a history of violent crime before carrying out his first known murders.
Before their deaths, Cooper had previously been charged with vehicle theft, being drunk and disorderly assaulting a police officer and assault occasioning actual bodily harm (ABH).
The murderer strikes again
Almost four years passed before John Cooper went on to kill again. This time it was Peter and Gwenda Dixon who would find their lives unexpectedly cut short.
The couple from Oxfordshire were on holiday and out walking along the coastal path in Pembrokeshire on June 29 1989 when they were killed. Their bodies were found along the path.
Like Richard and Helen Thomas, the Dixon's had been shot at point-blank range.
Before killing them, Cooper tied them up and demanded they hand over their bank card and pin details. He used this to withdraw money from their account from multiple cashpoints.
The police were able to determine that the same shotgun was used to kill the Dixons as had been used to kill the Thomas' nearly four years prior. That is when they knew they were dealing with a serial killer.
Cooper is imprisoned, but not for murder
Cooper went on to commit several other crimes. In 1996 he attacked a group of young people in Milford Haven, raping one teenage girl at knifepoint and sexually assaulting another.
The following year he attacked a women in her home in Pembrokeshire at gunpoint. He bound and gagged her but she managed to escape.
He was then arrested in 1998 for a series of home invasions and sentenced to 16 years in prison for 30 counts of robbery and burglary.
The cases are reopened
It was not until 2006, when Detective Superintendent Steve Wilkins reopened the Thomas and Dixon double murder cases, that Cooper was found to be a murderer as well.
The reopened investigation was called Operation Ottawa and used advances in technology and forensics to find new evidence.
Through this, Dyfed-Powys Police were able to link microscopic DNA and fibres from the murders to the burglaries that Cooper had perpetrated and was already serving time for.
DNA evidence revealed a drop of Peter Dixon's blood on a pair of khaki shorts taken from Cooper's home and the police managed to recover the shotgun used in both double murders.
In 2009, just a few months after Cooper was released from prison, he was re-arrested and later sentenced to four life sentences for the double murders in 2011.
He claimed his innocence but was denied an appeal.
How ITV's television gameshow Bullseye helped the case against Cooper
One of the unusual aspects of this story is that footage from ITV's darts-themed gameshow Bullseye was used in the case against Cooper.
The murderer had appeared on the show just one month before killing Peter and Gwenda Dixon. Pictures of him on the show matched a police sketch drawn in 1989.
On the show, Cooper talked about his expert knowledge of the Pembrokeshire coastline - where he would then go on to kill the Dixons days later.