It has caused a stir for doing what official inquiries failed to: naming the major suspects in the massacre, by cross-referencing a purported confession letter with the report by Police Ombudsman Dr Michael Maguire.

The revelations have shocked the victims’ families and sent a local man accused of opening fire into hiding with his wife.

The massacre in context

No Stone Unturned opens onto the Heights Bar in Loughinisland on 18 June 1994, when the small south Down village became the scene of one of the most brutal and senseless atrocities in the Troubles.

The village is instantly recognisable to all those familiar with Irish rural life, and was almost entirely untouched by the Troubles. A majority of residents might have fallen into the Catholic/nationalist category, but the village was almost entirely apolitical and content to remain so.

That night, a crowd of around 15 people was gathered to cheer Ireland to victory against Italy in the World Cup.

At 22:10, a red Triumph Acclaim, purchased two days previous in east Belfast, pulled up outside. Two men in boiler suits and balaclavas jumped out while another stayed behind. They produced a Czech-made vz. 58 assault rifle. One man held the bar door open while the other screamed “Fenian bastards!” and fired inside at random.

Eleven were shot in the back as they watched the football. Six men – Adrian Rogan (34), Malcolm Jenkinson (52), Barney Green (87), Daniel McCreanor (59), Patrick O’Hare (35) and Eamon Byrne (39) – lay dead. The young barman suffered a bullet to his kidneys. A witness heard the death squad laughing as they made good their escape.

The attack matched the modus operandi of loyalist paramilitaries throughout the conflict: not to target active militant republicans, but to terrorise the Catholic/nationalist population to cease giving support to republican paramilitaries or parties, drawing on the same military doctrine used by the British Army in Yemen and Kenya.

Exposing collusion and cover-up

The documentary sets out to reveal in explicit detail the actions taken by the RUC to deliberately frustrate the investigation. Evidence was improperly collected and frequently destroyed, glaringly obvious leads were ignored, and interviews were conducted in near-farcical manner.

Solicitor Niall Murphy of KRW Law, who represents the Loughinisland families, speaks of his disbelief on hearing the RUC had destroyed “the single largest physical exhibit in the case”, the Red Triumph.

The crushing of the car is shown repeatedly to reinforce the film’s principal point: the police weren’t negligent, but guilty of collusion. Balaclavas with hair samples were destroyed and records requested by the discredited 2009 Hutchinson inquiry had been incinerated.

Powerfully, a former RUC detective speaks on film of his disillusionment, blasting the “orchestrated” interviews he later had with the officers on the scene. The hidden hand of Special Branch is strongly felt.

But the film also takes a humorous turn detailing the exploits of Barry McCaffrey of The Irish News, whose investigative journalism is a source for much of the documentary. He speaks of tracking down and spooking the former RUC Senior Investigating Officer in his retirement in the south of France - as McCaffrey puts it: “He shit himself.”

The documentary leaves viewers with an overwhelmingly convincing case of collusion between the RUC and loyalist terrorists, with the full knowledge of senior members of the UK government.

The film finishes as it begins. Then-Northern Ireland Secretary, Sir Patrick Mayhew, stands outside the Heights Bar, asserting in the drawling upper-class tones of a Viceroy that “the RUC will never give up until the perpetrators of this heinous act are brought to justice”. Except at the end of the film, his statements takes on new significance.

The ongoing campaign for justice

No Stone Unturned will blow wind in the sails of the Loughinisland families’ campaign for justice for their loved ones. Already, the families have renewed their calls for “arrests, prosecutions, and accountability” in the wake of its release. Outrage, particularly in south Down, that the murderers who carried out the massacre were not only free but also living locally may also provide an impetus for the PSNI to take action.

What remains to be seen, however, is whether the current PSNI hierarchy decide to act on the revelations contained in the documentary. Over 3,000 murders that occurred during the Troubles are yet to be solved, and the task of selecting which cases to prioritise is a politically sensitive matter. There will be political resistance to the prosecution of loyalist terrorists and their former collaborators.

The PSNI, like other police forces, must decide whether to press charges on the merits of the evidence. Unlike other forces, it must also assess the political fallout their decision is likely to cause. Whether they choose to or not will be indicative of the maturity and stability of Northern Ireland’s peace process: can our peace survive the truth?

One hopes it can.