The three-part mini drama series follows the investigation of two double murders in the Pembrokeshire area in the 1980s.
Newly-promoted Detective Superintendent Steve Wilkins reopened the cold cases in 2005, but it wasn’t until 2011 when he was finally brought to justice.
With the help of new forensic methods, Wilkins and his team managed to link the murders and a string of burglaries committed in the 1980s and 90s.
The murderer, who was known as the Bullseye Killer, even appeared on a TV gameshow just weeks before his second double murder.
Here are the grisly details of Cooper’s crimes, which eventually earned him a life sentence in prison.
John Coopers crimes
December 22 1985
Cooper killed millionaire farmer Richard Thomas, 58, and his 54-year-old sister Helen at their manor house near the village of Scoveston.
They were bound and gagged before Cooper shot them, and set fire to their remote farmhouse.
Richard’s body was found with a point blank entry wound to the stomach.
June 29 1989
Cooper went on to murder Peter Dixon, 51, and his wife Gwenda, 52, from Oxfordshire on the final day of their summer holiday in the coastal village of Little Haven.
They were camping and taking their last walk along the coastal path before heading home, but were cornered by Cooper, brandishing a sawn-off shotgun.
Cooper tied the couple up, and demanded they hand over their bank cards, pin-numbers and cash.
They were shot at close range on the footpath, and Gwenda was naked from the waist down.
She appeared to have been sexually assaulted and Peter’s 22-carat wedding ring and wallet were stolen.
In March 1996, a man wearing a balaclava and brandishing a sawn-off shotgun approached five teenagers in Milford Haven.
Cooper was later sentenced for the rape of a 16-year-old girl, and a sexual assault on a 15-year-old girl in March 1996.
These were also at gunpoint in Milford Haven.
Cooper had a long history of crime, including 30 robberies and violent assault.
When the cold case of the murders was reopened, Cooper was already serving 14 years in prison for the burglaries.
This included robbing a woman with a sawn-off shotgun at her home in the village of Sardis.
He was eventually arrested for these crimes in 1998 after a botched robbery of a woman who managed to raise the alarm in her home.
Cooper threw his balaclava and gloves in a nearby hedgerow, which the police later recovered.
He was given parole in 2006, when the police started collecting evidence about the Pembrokeshire Murders.