With 19 murders between them, the Shankill Butchers were the most prolific gang of serial killers in UK history. During the dark days of the Troubles, their savagery stood apart, paralysing both communities in Northern Ireland with fear. With unique access to thousands of pages of evidence and exclusive interviews, Stephen Nolan goes back to the patch where he was brought up to ask how the Shankill Butchers got away with murder for so long.