IN 1973, Margaret Campbell saw her trade unionist husband Pat gunned to death on their doorstep as she stood by his side.

When Margaret attended an identity parade, she told the police where her husband’s killer was standing in the line up.

This was not good enough for the police, who insisted Margaret place her hand on him. She collapsed.

The alleged killer was in fact police agent Robin Jackson, known as the Jackal. No one has ever been convicted of Pat’s murder.

This chilling episode is just one of many vividly retold in Sean Murray’s latest film Unquiet Graves, which has just premiered at the Belfast Film Festival.

The documentary painstakingly maps out a sectarian killing spree that rocked counties Armagh and Tyrone at the height of the Troubles in the 1970s, claiming around 120 innocent lives.

With haunting reconstructions, heartbreaking survivor testimony and a sinister encounter with a whistleblower, Murray brings to the big screen the harrowing story of the Glenanne Gang.

It’s a cinematic masterpiece, poetically narrated by Oscar-nominee Stephen Rea, which builds on the investigations and litigation undertaken over the last four decades by a whole range of people.

We meet the victims’ families turned justice campaigners and their allies in the media, clergy, law firms and NGOs – and, occasionally, some good cops.

But it is a bad cop who steals the show. Murray captures on camera a rare interview with John Weir, whose unforgeable face resembles a living skeleton.

He is a disgraced Royal Ulster Constabulary (RUC) officer, with a cadence and stare that leaves the audience chilled to the bone. Weir was part of the Glenanne Gang.

He joined the assassins in despair at what he saw as IRA attacks going unpunished. His police colleagues wanted to “take the war to the IRA” and “fight terror with terror.”

These off-duty policemen teamed up with part-time soldiers to form a loyalist paramilitary death squad.

It was in response to one of their attacks that, in 1976, Republicans carried out the massacre at Kingsmill of Protestants on their way home from work.

Through Weir, we learn about what could have happened next — how the Glenanne Gang discussed a plan to “shoot up” a Catholic primary school.

That attack never happened, in part because the gang believed the plan was being fed to them from British Army headquarters.

Weir’s men worried that killing schoolchildren would have put the country in “a proper civil war situation.”

Paul O’Connor, a campaigner from the Pat Finucane Centre (PFC), says “military intelligence wanted the whole situation to spiral out of control”.

Murray draws heavily on the research by the PFC and their partner group Justice for the Forgotten, whose deep understanding of the conflict allows his film to explain not just what happened but why.

The British state has long sought to portray itself as a neutral arbiter in the Troubles, stuck in the middle between two warring communities.

Unquiet Graves pierces this myth and shows convincingly that Britain was a colonial protagonist in this conflict who used one community, the loyalists, as a deniable third force to carry out its dirty work while keeping its hands clean.

Unquiet Graves is scheduled to go on international release this year. If you'd like to organise a screening, visit unquietgraves.com