The team was set up by PSNI Chief Constable Sir Hugh Orde specifically to provide the relatives of the 3,268 people killed during the Troubles with answers about how their loved ones died and to seek out opportunities to gather additional evidence against those responsible.

In looking at Enniskillen now the team is treating the atrocity as an exceptional case. Its 100 officers have been dealing with the murders in chronological order. Starting with 1969 they are believed to have reached the early '70s. Had Enniskillen not been "exempted" from this rule and taken out of sequence the team's investigation of the events of November 8, 1987, would still be a long way down the line.

A source stressed: "There have been some meetings with families but this process is at the very earliest stage of review."

The team will be reviewing a case in which there is precious little evidence. Much of what is known about the attack is based on intelligence and the word of informers.

A former RUC officer with detailed knowledge of the investigation believes the bombing of Enniskillen was not the work of an individual unit but was carried out by a team made up of personnel drawn from IRA units in Enniskillen, Kinawley/Derrylin, Donegal and Monaghan.

"That indicates that it would probably have been organised at a higher level," he explains.

This was to be a major co-ordinated attack on the Remembrance Day services in Enniskillen and at Tullyhommon, on the Fermanagh/Donegal Border, where a landmine planted where children were assembling for a church parade failed to detonate.

For such a specialised operation the IRA would have drawn together its most trusted personnel.

According to the former RUC man components for the Enniskillen bomb were supplied by a senior figure from the South East Fermanagh and Monaghan Border area and it was assembled by an explosives expert in Donegal. It was planted in the former Reading Rooms beside the Cenotaph by members of the Enniskillen and Derrylin/Kinawley unit.

The former RUC man believes the location of the bomb points to at least one of those involved in the attack. He says that in 1972 an IRA unit was using the Reading Rooms as a base. One night while the terrorists were assembling an incendiary bomb it exploded prematurely and a number of people were injured. They went on the run and fled to Donegal where one of them became a central figure in the local IRA unit, bringing with him detailed information about the lay-out of the Reading Rooms where the bomb was placed and exploded to such deadly effect.

When the dead and injured had been pulled from the rubble and the dust finally settled police were left with very little physical evidence in terms of fingerprints, fibres and the signature marks of the bomb maker.

Detective Chief Superintendent Norman Baxter, who was involved in a major review of the case in 2004, says: "There were limited forensic opportunities due to the scale of the devastation."

Officers took statements from numerous witnesses.

"There was very, very good co-operation from the public," states DCS Baxter.

"But through stealth and good planning they (the IRA) were able to plant it with minimal opportunities for people to see them doing it," he explains.

He is convinced that there are people out there who know who was involved and have not come forward with that information, and he called on them to do so, even at this late stage.

Having examined the evidence in the case DCS Baxter is in no doubt about the bombers' aim.

"Their intention was to inflict casualties. The only mistake in the operation was that the bomb went off before the parade arrived. If they had killed 11 soldiers and 11 civilians it would have been a success. They would have said the 11 civilians were 'unfortunate casualties,' 'collaterral damage.' The victims were of no consideration," he states.

"Before the Historical Enquiries Team was set up we had carried out a full review of the investigation here in Enniskillen and a number of new lines of enquiry were identified and a number of new potential suspects identified," he adds.

All of that information was passed to the Historical Enquiries Team at the beginning of this year.

DCS Baxter says that in the immediate aftermath of the bombing the republican leaderships' only consideration was how to extricate themselves from what was a public relations disaster and minimise the damage to the organisation and the flow of money from America.

"My assessment, having reviewed the material, is that the leadership put in place a strategy to support the individuals within the units that carried out the atrocity. There was no exodus from the IRA in Fermanagh and Donegal over the incident and indeed some believe the death and destruction at the cenotaph was a success," he says.

However, the fallout from the bombing left the terrorists looking nervously over their shoulders.

"The public mood within republican and nationalist areas across Northern Ireland was one of revulsion and there was an erosion of financial and public co-operation in the weeks following Enniskillen which undermined confidence in IRA military operations where they had major concerns that they couldn't depend on being able to hide within communities for fear of the revulsion caused by Enniskillen leading people to pass information to police. There was a complete decline in violence over a period of months," recalls DCS Baxter.

In his view: "The bombing was an attack on everything decent in society; on unarmed civilians at prayer. If you see them(the IRA) as an army, it was a war crime. This was murder without remorse."

He believes the atrocity led to a rethinking of its strategy by the Sinn Fein leadership and eventually to the talks between party president Gerry Adams and John Hume of the SDLP and the beginning of the Peace Process.

Jim Dixon, one of the survivors of the bombing, has been highly critical of the police investigation, and Aileen Quinton, whose mother Alberta was killed in the explosion, has also asked questions about why police have failed to bring the perpetrators justice.

A PSNI spokesman said: "The police are as frustrated as all the families and all the victims that no-one has been made amenable. Obviously the police haven't suffered the same pain. There are a number of officers who have put a large part of their careers and lives into trying to get somewhere with this. Frustration and regret are the two continuing emotions."

DCS Baxter adds: "I can understand how they feel that the criminal justice system has failed them. It is not only the police that have failed them but society has failed them; people in their own community whose family members were involved in scouting the bomb in. The wider republican movement knows (who carried out the bombing)."

Even if the Historical Enquiries Team makes a breakthrough in the investigation and brings the perpetrators to justice, under the terms of the Good Friday Agreement anyone convicted of committing a terrorist offence prior to the IRA ceasefire of 1994 can be sentenced to no more than two years in prison.

A black time for republicanism

If the Enniskillen Bomb had been a deliberate attempt to murder civilians then it would have been political suicide for Sinn Fein, according to party spokesman Brian McCaffrey.

It undermined electoral support and set back republican political ambitions by 10 years, and 20 years later is something republicans are still "very uncomfortable" with.

"In all of the years of the republican struggle it's probably the blackest period there was, with the possible exception of the '81 Hunger Strike, and I think the period that most threatened the existence of that struggle," states Mr McCaffrey.

"It's still something that makes republicans very uncomfortable; maybe for some, even difficult to talk about, because they just find it very, very hard to deal with in terms of its scale and effect and maybe also from the point of view that there's a certain amount of collective guilt."

Mr McCaffrey was one of eight Sinn Fein members of Fermanagh District Council in 1987 and recalls his reaction to the scale of the carnage.

"I think it was basically the same reaction by all republicans: How could this happen? I can't actually recall anyone complaining directly to me about it but republicans and their supporters would have been absolutely shocked by it," Mr McCaffrey remembers.

"In the days following there seemed to be a push by politicians, media and some church people to portray it as a deliberate act against civilians. I think probably republicans at that stage would have been thinking 'this wasn't intentional'.

"I believe that most republicans felt it was unacceptable. It had put republicans under serious pressure and obviously this whole spin that this was a deliberate attack on the civilian population was, at the very least, undermining the struggle. The purpose of the spin was to finish off the republican struggle. To me this was a cynical exploitation of the tragedy. That was how I saw it at the time," he adds.

Mr McCaffrey recalls the pressure put on his party colleague and then council chairman, Paul Corrigan.

Mr McCaffrey believes the bombing set back Sinn Fein's political progress by 10 years.

He accepts: "A certain amount of our support base did walk away.

While support remained steadfast among those who believed the bombing was not a deliberate attack on civilians, many nationalists were not prepared to support Sinn Fein.

He says the seeds of what became known as the Peace Process were sown before the bombing but the explosion served to focus minds on the process.

Mr McCaffrey says that over the past 20 years he has spoken to unionists who would have accepted that republicans "were sorry and believed it shouldn't have happened".

"I think there's that collective guilt from all republicans in that they would feel it was totally unacceptable to attack ordinary people at a Remembrance Ceremony."

He adds: "Had it been something deliberate it would also have been political suicide. There will always be those who refuse to accept any kind of apology from republicans. I can't change that but that doesn't make our apologies any less genuine."