Further links are now being made between Welsh serial John Cooper and the murder of an elderly brother and sister from Pembrokeshire.

Forensic psychologist Dr Clive Sims said there are enough similarities between Cooper’s crimes and the deaths of Griff and Patti Thomas at their farmhouse in Llangolman in 1976 to suggest “he may at least be a suspect in this case.”

Cooper was jailed in May for the shotgun murders of Richard and Helen Thomas and Peter and Gwenda Dixon in the 1980s. Richard and Helen Thomas were a wealthy brother and sister living in a manor house near Milford Haven, while the Dixons were a married couple enjoying a holiday walk on the Pembrokeshire Coastal Path.

The deaths of Griff and Patti Thomas, aged 73 and 70, in December 1976 were initially treated as murder.

But Dyfed-Powys Police later concluded that Mr Thomas had argued with his sister before hitting her on the head with a blunt instrument, and setting himself on fire.

No weapon was ever found.

In February 1977 an inquest returned a verdict of manslaughter in the case of Miss Thomas and an open verdict for her brother. The pair had lived together all their lives.

Last week, on BBC Wales’s Taro Naw programme, Dr Clive Sims said he believed the inquest verdicts were “unsafe”.

“I feel that there would be substantial evidence for looking at this case again as a cold case,” he said, adding that he believes they were killed by an intruder following a “botched burglary”.

“There is an empty cash box, the bureau has been broken into, the back door is unlocked and certain aspects of it simply do not make sense,” he said.

It was reported that Cooper had no record of criminal activity at the time of the deaths at Llangolman in 1976.

However, that is not quite the case. During research for an hour-long special on Cooper, The Bullseye Killer, on ITV in May, the Wales This Week team discovered that Cooper had first come to the police’s attention in the 1960s.

Firstly, as a 17-year-old when he was caught with an offensive weapon and then for assault on police officers and a man outside a nightclub.

As Dr Sims said during Taro Naw: “It’s quite probable that he would have been offending earlier [before 1985]. It would be extremely unusual for someone to start that late in life on a criminal career, and to start with that level of violence.”

Perhaps the most convincing links between Cooper and a further suspicious death remain those with Florence Evans.

Mrs Evans, known as Flo, who was found fully clothed in the bath of her home in Rosemarket in February 1989, shortly before the Dixons were shot.

She was a 74-year-old widow, but was active and fit.

Friends and neighbours were deeply concerned by the strange circumstances of her death.

She had been found fully dressed and lying face down in a bath of freezing cold water.

There was no sign of forced entry at the house and an inquest recorded a verdict of accidental death.

However, Flo Evans employed a local gardener and handyman – John Cooper.

Dyfed-Powys Police officers are reviewing whether he was connected to any other murders, and are liaising with South Wales Police over any possibility “connectivity” with the deaths of Harry and Megan Tooze who were shot at close range at their farmhouse at Llanharry near Bridgend in 1993.

John Cooper is one of the most notorious criminals in British history.

He burgled, raped and murdered during a 15-year reign of terror, which included four executions by shotgun. And, far from keeping a low profile between crimes, he even took time to appear on the gameshow Bullseye.

This year, he was jailed for life. ITV Wales had exclusive access to the police enquiry and forensic science which finally trapped Cooper.

An hour-long programme The Bullseye Killer was transmitted the night Cooper was jailed in May. But it’s still on-line. Follow this link to watch it now.