Susanna Reid gains privileged access to police interview tapes from the investigation into one of Britain's most prolific serial killers, John Cooper, who murdered four people in Wales. After brutally murdering the quartet in Pembrokeshire, 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.This programme tells how the cold case team who picked up the investigation discovered that four years after murdering the first couple in 1985, he had appeared on gameshow Bullseye, which was weeks before the vicious shooting of another couple taking a coastal walk on the last day of their Welsh holiday.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