It was a warm August Saturday afternoon and Market Street in Omagh was thronged with shoppers and tourists. With only one weekend to go before the schools reopened after the summer holiday, many parents had brought their children into town to buy new uniforms.

There was an international party on a day trip from across the border, among them Spanish children on a language course, milling through the town. The atmosphere was festive.

Unfortunately, a group of four men had macabre intentions that would shatter Omagh’s gaiety and cast a terrible shadow over the town for more than a decade.

Michael McKevitt, Liam Campbell, Colm Murphy and Seamus Daly had planned for the moment. The men’s splinter organisations had been carrying out small-scale bombings across the Province in an attempt to undermine support for the Good Friday agreement. Now was their opportunity to commit a mass murder.

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It was about two o’clock when a maroon Vauxhall Cavalier — stolen a few days earlier south of the border and fitted with Northern Ireland registration plates — pulled up outside S.D. Kells, the school uniform shop.

Two men got out and walked away. Only a trained eye would have noticed anything suspicious about the car; the way that its chassis sat low on the rear wheels, giving away clues to its deadly cargo.

A 300lb fertiliser bomb primed with a Semtex trigger was in the boot, armed to explode with a timer that one of the men in the car had set running before making his escape.

At 2.30pm three telephone warnings were made in quick succession, two of them from the same telephone box at McGeough’s Crossroads, Forkhill, South Armagh, and the third from another phonebox in Manorhamilton, also South Armagh.

The calls gave confusing and misleading information but all three were verified for authenticity with a codeword: “Martha Pope” — the same that had been used in a Real IRA bomb attack on the Co Down town of Banbridge a few weeks earlier.

One call simply said: “Bomb. Omagh Town. Fifteen minutes”. The other two mentioned the courthouse as a target and said that it would explode in 30 minutes. One call also mentioned “Main Street”, which does not exist.

Police swiftly cordoned off the courthouse and began moving the public away, shepherding people to what they thought was safety but which turned out to be the most lethal place — the lower part of Market Street.

Even today bomb warnings in Northern Ireland are still part of everyday life and people in Omagh on August 15, 1998, reacted as they always did, with resignation, bemusement but little anger or panic.

That only set in after 3.10pm, when the bomb exploded. Survivors recalled the eerie silence straight after the detonation, which created a fireball that reached temperatures of 1,000F (540C) and sent shrapnel flying for 300 yards.

A holidaymaker captured some of the terrible scenes on his video camera: the desperate scrabbling of rescuers at collapsed buildings, bloodied survivors reeling drunkenly away from the seat of the blast, some being given first aid, some screaming.

Of the 29 victims — 31 when the twin baby girls almost at full term being carried by their mother are included in the murder toll — 21 died immediately. Some bodies were never recovered because of the victims’ proximity to the bomb. About 300 people were injured, including some who lost limbs or their sight.

The inquests later heard descriptions from medics of battlefield conditions. A leisure centre and an army barracks were turned into emergency centres and a temporary mortuary.

Such was the scale of the massacre that local hospitals could not cope. James Barker, a 12-year-old boy who was on a school trip from Co Donegal, would have survived had he been given immediate treatment, a surgeon told his inquest, but medical staff were overwhelmed.

Three days later the Real IRA claimed responsibility, blaming the security forces for the deaths because they had not heeded its three clear warnings. Mo Mowlam, the Northern Ireland Secretary, called that “a pathetic excuse for mass murder”.

Five weeks later the Royal Ulster Constabulary and An Garda Síochána arrested 12 men in connection with the bombing, but subsequently released all of them without charge.

In February the following year seven men were arrested. One of them, Colm Murphy, was charged in relation to the bombing. In January 2002 he was convicted at the antiterrorism Special Criminal Court in Dublin of conspiracy to cause an explosion and given a 14-year sentence. Three years later his conviction was quashed on appeal when it was revealed that two police officers had falsified interview notes.

The Royal Ulster Constabulary was roundly criticised by the Northern Ireland Police Ombudsman, which published a critical report in December 2001 claiming that police had ignored previous warnings and failed to act on vital intelligence.

The report had been prompted by a former agent inside the Provisional IRA using the pseudonym Kevin Fulton, who claimed that he had given his MI5 handlers key information three days before the Omagh bombing. Similar allegations were made the the handling of intelligence by police in the Irish Republic. “The victims, their families, the people of Omagh and officers of the RUC were let down by defective leadership, poor judgment and a lack of urgency,” the ombudsman’s report said. Sir Ronnie Flanagan, the chief constable, said that he would “publicly commit suicide” if the allegations were true.

The families of the victims had already become frustrated at the failure to bring succesful prosecutions. It was James Barker’s father, Victor, a Surrey solicitor whose family had moved to Co Donegal in search of a better life, who had the idea of the a civil action against five individuals. But that decision was taken only after it became clear that the words of Tony Blair, then the Prime Minister, who pledged to leave no stone unturned in the quest for justice, rang hollow.

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