Footage from the television game show Bullseye in May 1989, in which he appeared as a contestant, was later used as evidence against him, comparing his image with a sketch of a suspect in the Dixons' murder.

Because of subsequent developments in DNA and forensic science, the police carried out a cold case review in April 2009 and were able to identify Cooper's shotgun as being the murder weapon.

"[9] On 22 December 1985, Cooper targeted a three-storey farmhouse at Scoveston Park, killing brother and sister Richard and Helen Thomas, and then burning down the house.

[10] On 29 June 1989, Peter and Gwenda Dixon were on holiday in Pembrokeshire and were due to take their last walk along the coastal path when they failed to return.

[10][11][12][13][14] Cooper had tied the couple up, demanded they hand out their bank card and then forced them to disclose their personal identification number (PIN).

Cooper, carrying a sawn-off shotgun, robbed Peter Dixon of £300 and shot the couple in the face at point blank range.

[6] Footage from an edition of the ITV gameshow Bullseye recorded on 28 May 1989, on which Cooper was a contestant, was later used to match him to a sketch made from witness descriptions.

On 24 May 2016, the Welsh language television channel S4C broadcast a documentary in the series Y Ditectif (The Detective) about the way in which evidence against Cooper was gathered using the latest forensic techniques available at the time, the strategy used by Dyfed-Powys Police in interviewing him and his eventual conviction.

[25] On 12 July 2018, a documentary about Cooper, named The Gameshow Serial Killer: Police Tapes, was aired by ITV as part of the channel's 'Crime and Punishment' season.

[33][34] Flo Evans, a 72-year-old widow, died soon after Cooper murdered Peter and Gwenda Dixon in 1989, being found fully-clothed in a half-full cold bath in her cottage.

[34] She was unexpectedly mentioned by Cooper in his trial as part of his own defence evidence, and during interviews he discussed how he had been in her house, when detectives were already aware of her suspicious death.

[37] Some similarities with Cooper's known murders were noted, including the fact that both victims were shot at close range and attempts made to hide their bodies.

[37] In 2011 a forensic psychologist, Clive Sims, claimed to the BBC that the deaths of an elderly brother and sister at their farmhouse in Pembrokeshire in 1976 could be linked to Cooper.

[40] Over many years, public campaigning has taken place on the basis that the inquest verdicts were not safe and that there was a case for further investigation to establish whether a third person may have been responsible for the deaths.