It is a Sunday night in the pub. There is football on the TV and people packed around the bar - then suddenly, violence erupts. Bottles are smashed and broken glass is slashed across the throat of one of the biggest men in the bar. The melee spills out into the street, where the injured man says out loud the name of the man he believes attacked him.

There is, he says later, an immediate reaction with hostile men in the crowd around him saying: "You're getting it, you're getting it." A knife flashes and two friends are left bleeding heavily on the pavement. One of them is dying.

Robert McCartney, an engaged father-of-two, took no part in the violence, according to the few available accounts of what happened that night in January 2005.

But the 33-year-old ended up losing his life, and no one has ever been held to account for his murder – an outcome that reverberated across the world and is now the subject of a documentary series, Murder in Belfast: Sisters Seeking Justice. The failure to secure convictions can be linked to the organisation which exercised power over many of those present.

Even while Robert McCartney was being treated by paramedics on the street, his killers' tracks were being covered.

Back in Magennis's bar, right behind the Royal Courts of Justice in central Belfast, the pub's CCTV tape was snatched and the bar was cleaned with bleach.

"It wasn't their first rodeo," said David Lowans, a former Police Service of Northern Ireland (PSNI) detective who worked on the case.

"They were experienced in clearing themselves forensically." The series, made by BBC Northern Ireland in association with Irish broadcaster RTÉ, obtained crime scene video and photos showing the aftermath of the incident. The clean-up wasn't complete and police were able to find some blood stains in parts of the bar. But substantial evidence seemed to have gone. The knife used in the murder was never found and reportedly destroyed.

Crucially, witnesses appear to have been silenced "Out of the 70 people who were in the bar, we got no information from anybody to say the detail of what happened," said Lowans.

"We either got: 'I was in the toilet' or 'I didn't see anything', which is unbelievable.

"Whenever this altercation took place there were lots of people there." In the days after the murder, Robert's five grieving sisters and his devastated fiancée, Bridgeen Hagans, had even less information about the murder than the police.

"We were still completely in the dark as to what had happened," said his sister Paula McCartney.

"Didn't know where it had happened, how it had happened, who'd done it. Nothing." But they soon began to hear rumours.

"It was just people coming in and telling what they'd heard in the street and 'such and such was in the bar that night'," said another sister, Catherine McCartney.

"All the talk about who'd done it, who was involved." The information they were receiving explained why some witnesses weren't talking. The killers were said to be members of the Provisional IRA. The IRA had waged a campaign of violence for 30 years.

In 2005, as Northern Ireland's peace process was being cemented, the group was observing a ceasefire.

It issued a statement denying involvement in McCartney's murder, but that didn't convince witnesses who feared deadly consequences if they named IRA members to the police.

"I've been about these areas enough to know how dangerous this organisation can be when they want to be," said Ed Gowdy.

He was McCartney's friend and one of the witnesses telling his story on television for the first time. "I do know exactly the treatment informers get throughout the years. "If I tell what happened, am I an informer?

"My mate's dead, heartbreaking. But I had to survive what had happened here." Gowdy was one of several witnesses who were then questioned by the IRA, conducting its own secret investigation running parallel to police inquiries.

Another witness, who was also interrogated by the IRA, gives for the first time on TV a personal account of what happened in the bar.

The McCartney sisters, meanwhile, found themselves fronting a campaign for justice that increasingly drew attention to the case. All five sisters were interviewed for the series.

"It was my brother, and I just felt the world needs to know... what has happened to him," said Paula.

"I think [it was] the way we came across on TV, down to earth, working-class women, standing up for justice," said Gemma McCartney, the eldest sister.

"And doing something, breaking away from the old custom of not reporting these things to the police." Their campaign produced some extraordinary results, forcing the IRA to acknowledge several of its members were involved in the murder – though it said the killing was not done on behalf of the IRA.

The IRA said three members were expelled from the organisation. It even claimed it offered to kill them – although the McCartney sisters say that offer was never made.

The sisters' campaign caught the attention of America's then Special Envoy to Northern Ireland, Mitchell Reiss.

He recognised an opportunity to put pressure on the IRA to make moves that would complete the peace process.

"Our feeling was that if the McCartney family was willing, and it didn't seem exploitative, that they would be a very attractive poster for the criminality, the vigilantism, the injustice, the rough justice that was taking place across republican communities by the IRA," he said.

Just two months after Robert McCartney's murder, the sisters and his fiancée Bridgeen were invited to the White House for St Patrick's Day, where they met the then President George W Bush.

Northern Ireland politicians were kept out of the event, an exclusion that was widely interpreted as being aimed mainly at the IRA's allies in Sinn Féin.

Mitchell Reiss tells the documentary series that advances which followed in the peace process – including the IRA's announcement of an end to its armed campaign – were "a direct consequence" of the McCartneys' campaign.

In a statement, then Sinn Féin president Gerry Adams credits negotiations and amendments to policing and justice arrangements as key drivers in completing the peace process.

In the aftermath of those advances, the intense attention on the McCartney sisters' campaign began to fade. But, as the documentary shows, their pursuit of justice was not over. In 2008, three men were brought to trial and acquitted.

Murder in Belfast: Sisters Seeking Justice uses original court material to bring two key witnesses and the McCartney sisters back over the dramatic evidence from that trial.

All episodes of Murder in Belfast: Sisters Seeking Justice will be available on the BBC iPlayer on Sunday 25 October.

The series will begin on BBC One Northern Ireland that evening at 22.30 GMT.