Image caption Stephen Nolan returns to Belfast's Shankill Road to ask why the the murder gang remained free for so long

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.

The Shankill Butchers is a new documentary in which Court Service files from the trial are released for the first time to BBC Northern Ireland.

Stephen Nolan hears accounts from victims' families speaking for the first time and questions the former CID chief in charge of the investigation.

The programme also helps to build a psychological profile of the "ruthless and sadistic" gang leader, Lenny Murphy who, even though jailed for six years for an unrelated offence, would continue to direct the murders from his prison cell.

Image caption Bobby 'Basher' Bates, a member of the Shankill Butchers gang, was shot dead in 1997.

Using a combination of archive footage and contemporary interviews, The Shankill Butchers, looks back at some of the worst days of the Troubles and captures the complex political, social and religious climate in which these barbaric murders took place.

Robert McCann, the brother of victim Stephen McCann, remembers the second the 'lights went out'.

"I got up the following morning, knew nothing about anything except that my sister Sheila and my Auntie Sheila were sitting at the edge of the bed," he said.

"I remember very, very well putting on a football boot and just very casually asking, 'Oh, is he OK?'

"And she said, 'Just stop. He's dead'."

His sister, Delia McCallum, still can't bear to think about it.

The documentary also captures a tangible sense of the terror that swept across the city of Belfast.

Charlotte Morrisey, daughter of victim Joseph Morrisey, described it as the most fearful time of her life.

The very personal and private testimonies of the families of the victims leave no doubt about the legacy left by the brutality of the events.

Speaking about the horrific circumstances of her father's death, Charlotte said: "After I had been told it was my father, I looked over to my mother and she was holding herself rocking back and forward in the chair.

Image caption William Moore received 11 life sentences for his part in the murders

"She was just crying, 'Jesus not my Joe, not my Joe'.

"Her Joe was gone and I kind of knew by looking at her that it would probably have been kinder if God had taken her then... this wonderful vibrant woman had gone, all gone."

She added: "My understanding of it was that had these victims been Protestants this would never have been allowed to go on for as long as it did.

"Basically, it said you are like dogs on the street. You are second class citizens, you don't matter anyway.

"This is our country and if we want to take anyone out and we want to cut them to pieces, that's fine. We'll do that. We have done it, look at us, we're getting away with it."

Stephen Nolan questions Detective Chief Inspector, Jimmy Nesbitt, the former head of CID at Tennent Street Police Station and asks if enough was done at the time to stop the murders.

He puts it to him that some in the Catholic community felt a "blind eye" was being turned and that the identity of the Shankill Butchers was well known in journalistic circles, by British Intelligence and by many in the community on the Shankill Road.

Refuting the claim, former DCI Nesbitt replies: "That's absolute nonsense. These people were killers, they were killing innocent, purely innocent victims, people who were involved in nothing.

"Brutality, savagery, horrific killings and we wanted to catch them and we put every effort into catching them."

Image caption Sam McAllister was also a member of the notorious Belfast murder gang

Baroness May Blood also speaks of the shame brought to the Shankill community and how people had been silenced by terror.

"I would imagine 30 or 40% (of those living in the Shankill) knew who the butchers were, but they weren't going to name them," she said.

"These people had such a grip on the community and there was such fear you didn't cross them.

"I often wonder did the leaders of the UVF and UDA know what was going on. Was this being done in their name? You just have your own thoughts about it."

Stephen Nolan, aged six at the time the Shankill Butchers were convicted in 1979, said: "This is a story of wickedness and of human tragedy that is hard to comprehend.

"So many people died needlessly and I have nothing but admiration for the brave families of the loved ones who lost their lives in such terrible circumstances.

"In participating in the programme they have created powerful lasting testimony.

"I wasn't old enough to understand what was happening at the time and so making the documentary has been a journey and a learning experience for me.

"The Shankill Butchers used the Troubles as a cloak for what they did, and what they did was no more than serial killing."

Other contributors to the programme include the state pathologist at the time, Professor Thomas Marshall, psychologists Professors Peter Hepper and Geoffrey Beattie and journalists Jim Campbell, who campaigned at the time for the capture of the Shankill Butchers, and Deric Henderson, who reported on the trial at Crumlin Road Courthouse.

The Shankill Butchers is on BBC One Northern Ireland on Monday, 28 March at 2100 BST.