Peter and Gwenda Dixon were enjoying a hilltop walk in Pembrokeshire in 1989 when they were accosted, tied up and executed at point-blank range with a shotgun. In May this year their killer, John William Cooper, was found guilty of their murders and the 1985 slaying of siblings Richard and Helen Thomas. Real Crime looks at the forensic work that finally brought Cooper to justice. There are so many victims. Cooper had tried to shift blame for the Dixonsí murders on to his brutalised son, who says: ìOf all the things that he ever did to me, that was the lowest.î About this programme New series. The documentary returns, beginning with the case of John William Cooper, who murdered four people in Pembrokeshire in the 1980s. The crimes remained unsolved for more than 20 years, but Cooper was convicted in May following painstaking research and forensic analysis, which included the use of footage from his 1989 appearance on game show Bullseye.