“I am not a murderer. I don't care whether you believe it or not, I am not a murderer.”

Those were the words of serial killer John Cooper spoken to detectives interviewing him over the brutal murders of four people in Pembrokeshire.

Cooper's words are recalled this evening (Thursday) in a new ITV crime documentary, The Gameshow Serial Killer: Police Tapes.

As part of ITV’s Crime & Punishment strand, Susanna Reid gains privileged access to police interview tapes from the investigation into one of Britain’s most prolific serial killers.

Cooper killed brother and sister Richard and Helen Thomas at Scoveston in 1985 and Husband and wife Peter and Gwenda Dixon on the coast path near Little Haven in 1989.

After brutally murdering the four, Cooper eluded prosecution for the crimes for 20 years until a combination of a new detective team and advances in forensic science finally brought him to justice in 2011, when he was convicted for the killings.

The programme tells how the cold case team who picked up the investigation discovered that four years after murdering Richard and Helen Thomas, Cooper had appeared on TV gameshow Bullseye - just weeks before the vicious shooting of Mr and Mrs Dixon.

With much of the crime scene evidence long gone, and no witnesses to help the enquiry, nearly everything hinged on whether the detectives could find the key to make him admit the truth about his murders.

Susanna interviews the leading detectives in the case, some speaking for the first time, who tell her how they approached the investigation into their suspect, who was known to police but had never before talked under police questioning.

Det Insp Louise Harries, specialist interviewer, said: “This is the person who has committed these horrendous offences. And here he was sitting in front of me.

"What struck me was his piercing eyes. I think he was sussing me out, sort of glances, sideward glance from him really, he was probably trying to get the measure of me then. As much as I was doing the same to him.”

The Gameshow Serial Killer: Police Tapes, ITV tonight (Thursday), 9pm.