On the evening of 18 June 1994, the Republic of Ireland beat Italy in one of the opening games of the World Cup in Giants Stadium, New Jersey. The surprise victory unleashed wild celebrations in homes, bars and streets on both sides of the Irish border, precipitating a short-lived but intense bout of World Cup fever. In the village of Loughinisland in County Down, Northern Ireland, though, the night is enshrined in local memory for very different reasons.

About five minutes after Ray Houghton scored the single deciding goal, three men in balaclavas burst into the Heights Bar and opened fire on those who had gathered there to watch the game. Eleven of the 24 men present were shot in the back; six of them died outright. The oldest, Barney Greene, was 87; the youngest, Adrian Rogan, was 34. A survivor described bodies “piled on top of each other on the floor” of the small public bar. Other witnesses said that they heard the sound of laughter as the gunmen ran from the scene.

A few hours later, as distraught relatives were still arriving at the pub in search of their loved ones, the Ulster Volunteer Force (UVF), a notoriously ruthless loyalist paramilitary group, claimed responsibility for the murders. Although it was one atrocity among many in the Troubles, the Loughinisland attack is different for two reasons: it occurred just a few months before the ceasefire declared by paramilitaries on both sides; and it remains unsolved despite a welter of evidence identifying the suspects and linking them to members of the security forces.

The Loughinisland massacre is one of several murders that hint at the full extent of the collusion between the British state and loyalist paramilitaries in the province over the course of the Troubles. The failure to bring the suspects to justice is seen by many campaigners and activists as another attempt to prevent the evidence for that collusion from ever seeing the light of day – so damaging would it be to the reputation of both the security forces and the British government who sanctioned their actions.

“I didn’t want to pretend to be somebody who knew the last word about the Troubles,” says Alex Gibney, director of No Stone Unturned, a new documentary about the Loughinisland killings. “To me, the point of entry was the murder: what happened and why was there a cover-up? That may seem like a simple perspective, but it has a much bigger resonance both inside and outside of Northern Ireland. The state is meant to protect all the citizens in its jurisdiction, so what happens if the state is either enabling or covering up crimes?”

Gibney is known for taking on shady institutions and his approach – sheer doggedness underpinned by deep research – has earned him a reputation as one of the finest documentary film-makers of our time. He was Oscar nominated for his 2005 documentary, Enron: The Smartest Guys in the Room and made the riveting Going Clear: Scientology and the Prison of Belief in 2015. His range is impressive – he has tackled WikiLeaks and al-Qaida, James Brown and Frank Sinatra.

Watch the trailer for No Stone Unturned

In person, he exudes a quiet intensity that suggests he is simultaneously engaged and a little impatient to be elsewhere. No Stone Unturned is, it turns out, a more personal project. It was born out of his seemingly instinctive connection with the people of Loughinisland, whom he first met years before when filming a short film entitled Ceasefire Massacre. “It was this small village that still seems traumatised by the one terrible event that had happened there during the entire 30 years of the Troubles. And nobody will pay them the attention they deserve.”

At first, Gibney, a New Yorker, placed the atrocity in a distinctly American context. “I had it all wrong,” he says, shaking his head ruefully, “I initially thought of it as an anomalous attack by a wacko who, for some warped reason, carried out this dreadful crime. But then, I kept thinking, why this town, why that pub? That’s what drew me in, but the more we investigated, the more we saw that the beautiful countryside all around the village was crawling with nastier elements who were prepared to do these heinous things to innocents.”

No Stone Unturned begins dramatically with a chilling reconstruction of the killings. We see the full horror, in slow motion, the disposal of the murder weapons in a field nearby, before the cinematic evocation suddenly gives way to a montage of actual crime scene photographs. It is a heart-jolting juxtaposition made all the more real by the fractured eye-witness testimony of Clare Rogan, wife of victim Adrian, who recalls “bodies, the smell, the smoke lingering, broken glass ... a scene of carnage”.

Aidan O’Toole was 15 years old and working as a part-time barman in the pub when the gunmen burst in

The morning after the killings, Patrick Mayhew, the secretary of state for Northern Ireland, faced the local television cameras, addressing the killers directly in his patrician tones with the warning: “The RUC never give up – you will be caught and spend long years in jail.” Twenty three years later, that promise has yet to be fulfilled.

As shocking as the killings are, Gibney’s real subject is the aftermath. He finds out early on who the suspects are and slowly and painstakingly accrues the evidence that ties them to the murders. In the process, he lays bare the full extent, and the terrible cost, of the murky contracts made between the British authorities and the paramilitary killers they recruited as agents.

One of the strengths of his film is that we share Gibney’s mounting sense of disbelief as he interviews various insiders who catalogue the evidence of a cover-up: the botched forensic investigations, the disappearance of written records of interrogations, the evidence that has been mislaid. There is a sense throughout that the killers and the police officers complicit in their murderous activities acted with what can only be described as a casual arrogance, so confident were they that their collusion would not be uncovered.

Gibney finds out early on who the suspects are and slowly accrues the evidence that ties them to the murders

One detective describes the evidence as “a forensic goldmine”, citing how the gang abandoned the stolen car in a field close to where one of them lived without even bothering to burn it. The weapons were found soon afterwards having been thrown over a hedge nearby. Bafflingly, the police destroyed the car soon afterwards and later, when lawyers asked for the transcripts of the related interrogations, they were told that they too had been destroyed following a supposed asbestos scare in the building where they were stored. A former RUC officer from Liverpool, still sounding bemused, recounts how one suspect was released after he made a verbal promise to his interrogators that he would not carry out any more killings.

For Gibney, this on-camera testimony must all have seemed almost too good to be true. “Well, yes and no,” he says, pausing to gather his thoughts, “The thing is, the more you dig into the nature of their collusion, the darker it gets. One of the contributions the film makes is to put names to the players, not just the suspects but some of the police. There were decisions that were made not to pursue charges – even when they had one person confessing to a crime. Then, there was the whole issue of killings that were carried out by people who were operating as members of the security forces.” He pauses and shakes his head. “It was one of those situations in which everything became corrupt.”

Outside The Heights bar on the night of the shooting. Photograph: Belfast Telegraph/PR

Does he give any credence to the establishment view, as voiced in his film by the former secretary of state for Northern Ireland Tom King, that these difficult compromises were made in order to save lives; that the killers were rogue elements of the kind you would find in any conflict? He smiles ruefully. “One thing I have learned as a documentary film-maker is that we have a tendency when faced with horrific acts to say: ‘That was done by a bad person.’ The guards at Abu Ghraib were bad apples. So too were the UVF men who killed innocent Catholics. But, that’s the wrong part of the metaphor. They were part of a rotten barrel. When you make the barrel rotten, the apples rot, too. What I’m saying is, if you put people in those kinds of situations, they do bad things. And, not only that, but you also surround them with a context where it is now OK for them to do those bad things. That’s what happened here and that’s why there is this silence about the role of the state that until now has prevented the relatives from getting answers, from getting justice for their loved ones.”

There were decisions that were made not to pursue charges – even when they had one person confessing to a crime Alex Gibney

There are many surprising moments in Gibney’s film, but none more so than when a local councillor, Patsy Toman, produces a handwritten letter he received from a woman in the wake of the murders. It reads like an apology written in a moment of emotional crisis, but in court would have been deemed an admission of guilt by association. She admits to being a party to the planning of the crime and to knowing all the suspects. Toman passed the letter on to the RUC at the time but, though the woman and her husband were taken in for questioning, both were released. Gibney names the husband as one of the killers and reveals that the man lives with his wife in the nearby village of Clough, a loyalist stronghold, where they run a cleaning business that also specialises in the extermination of pests.

What is truly chilling, though, is Toman’s reluctance to read the full text of the letter. He hesitates when he gets to the names of the killers, then hands the letter to Gibney, his silence telling of the climate of fear that attended the long years of the Troubles and that still hovers around Loughinisland. It transpires that Toman’s house had been the target of a UVF bomb attack the year before the pub killings. “If they came after me once, they’ll come after me again,” he tells Gibney.

‘If you put people in these situations, they do bad things’ … Alex Gibney. Photograph: Larry Busacca/Getty Images

Throughout the film, the dignified testimony of the survivors and relatives of the victims hits home with the full force of a collective trauma. Aidan O’Toole was just 15 years old and working as a part-time barman in the pub when the gunmen burst in. He was shot in the kidney as he ran for cover amid the bullets and breaking glass. Twenty three years later, his face is a landscape of pain and bewilderment.

“I was one of the lucky ones – if you can call it lucky,” he says, haltingly, struggling to hold back tears as he recounts the horror of that evening. Like the others, he still struggles to make sense of why the UVF chose his village as a target. “You would never have doubted anyone around here,” he says of the mixed community he grew up in, his use of the past tense an implicit acknowledgment that all has changed since that summer evening in 1994.

During the making of the film, the Northern Ireland police ombudsman Michael Maguire, who had quashed the results of a previous inquiry into the Loughinisland killings, announced his own findings, stating, “I have no hesitation in unambiguously determining that collusion is a significant feature of the murders.” Gibney’s film goes several stages further, naming the suspects and their enablers and, one suspects, proving considerable embarrassment to the British government, security services and the RUC.

What does he hope his documentary will achieve? He thinks about this for a moment. “There is no statute of limitations for murder as far as I know, so that the perpetrators be arrested. That was one of the motivations of naming them. But, it’s about more than that. People say: ‘We have peace in Northern Ireland now, why rake up the past?’ But, when you speak to the relatives and the survivors, you realise that the past is the present. It can never be put to rest when it is shrouded in silence and secrecy.”