Evil John Cooper killed his first victims, brother and sister Helen and Richard Thomas, at their manor house near Milford Haven in 1985.
Four years later, he murdered Peter and Gwenda Dixon on a coastal path near Pembroke.
But when Detective Superintendent Wilkins took over the serious crime unit at the Dyfed-Powys police force in 2006 the murders were still unsolved – despite years of hard work from a determined team of detectives.
While they knew Cooper could be behind the chilling crimes, they didn’t have the evidence they needed to lock him up and it was a fight against time before he killed again.
And then, after more years of searching, they spotted something – a trophy Cooper stole from one of his victims which proved to be the crucial piece of evidence they needed.
At the time, Cooper was serving a 14-year term for a string of violent burglaries in Pembrokeshire.
Mr Wilkins said: “I knew he had to be a contender. The problem was at that time he was about to be brought for parole and it was clear to me that whoever committed these offences enjoyed what they were doing.
“If it was Cooper, who we knew was a gambler, we feared he’d soon be back into that cycle of offending. If he was the killer, he would kill again.”
It was Cooper’s wife Pat, who put up with years of abuse at the hands of her cruel husband, who unknowingly provided the piece of evidence that brought him to justice.
A police sketch from 1989, known as the “wildman” sketch, depicted a man seen using Peter Dixon’s cash card at an ATM after his murder.
A policewoman noticed some khaki shorts seized from Cooper bore a resemblance to those in the sketch, and also appeared to be women’s.
They turned out to be Gwenda Dixon’s, kept as trophy by Cooper, whose unsuspecting wife turned them up – and in doing so trapped vital DNA evidence in the seams.
Mr Wilkins said: “We found the DNA of Peter Dixon’s daughter trapped in the seam.
“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 tied Cooper to an attack on five teens in Pembrokeshire in 1996, who were confronted by a man in a balaclava brandishing a sawn-off shotgun who demanded cash and subjected two girls to serious sexual assaults.
And his appearance on the popular TV game show Bullseye also helped police pin him down.
At first it was thought show footage would be of little use as it was broadcast a year after the Dixons’ murders, but ITV news journalist Jonathan Hill worked with DS Wilkins and found it was filmed in May 1989, weeks before the killings.
Mr Hill told the Mirror: “Our archivist found it and I could not believe what I was seeing.
“The sketch was just from over his shoulder, and when I put it next to Cooper in the same position on Bullseye it was just a dead ringer.”
Mr Wilkins said: “In 33 years of policing I have never seen anything match as closely as that – you just look at it and you know that’s him.”
Cooper was arrested in May 2009, and jailed for life two years later.
The case has been turned into a three-part ITV drama starring Luke Evans and Keith Allen.