S4C DOCUMENTARY Y Ditectif (The Detective) will explore the techniques police used to interview serial killer John Cooper in the programme to be aired tonight (Tuesday).

In the third episode of the series, Y Gwyll/Hinterland actress Mali Harries meets the detective who was tasked with interviewing one of Wales’s most notorious killers.

Five years ago, John Cooper was given four life sentences for the 1985 double murder of brother and sister Richard and Helen Thomas at Scoveston, and the 1989 double murder of Peter and Gwenda Dixon on the coast path near Little Haven.

Both double murders cast a long shadow over Pembrokeshire.

It took two decades to convict the Milford Haven man, born in 1944, who had a history of crime and had previously been imprisoned for burglary and violent offences.

At Swansea Crown Court, in May 2011, he was also convicted for two serious sexual offences which he carried out at knifepoint, including the rape of a 16-year-old girl near the Mount Estate in Milford Haven.

During the programme, DS Gareth Rees discusses the techniques used to interview the murderer who will never be released on parole.

DS Rees, who retired last year as a detective sergeant in Cardigan, said: “I’ll never forget the feeling and what happened on the day I first came face to face with Cooper.

“The first time I saw him, he looked at me straight and said ‘have I met you before?’.

"I said ‘No, I don’t think so’, and he said ‘I’m glad about that’.

"He was a very cold man, very clinical and he didn’t show any emotion.”

In the programme, Mali finds out how the local man evaded the police for so long and how his appearance in 1989 on the popular game show Bullseye, hosted by Jim Bowen, in the long run helped Dyfed-Powys Police close the net.

Y Ditectif will be shown with English subtitles at 9:30pm on Tuesday, May 24.