Omagh was the biggest loss of life in a single attack in the Troubles
An official report has rejected claims that vital intelligence about the Omagh bombing was deliberately held back.
Intelligence Services Commissioner Sir Peter Gibson said information on the bombers was shared with police, but could not have stopped the 1998 attack.
In his report, Sir Peter said details from telephone intercepts were passed on "promptly and fully" and in accordance with proper procedures.
Twenty-nine people were killed when a Real IRA car bomb exploded in the town.
A BBC Panorama programme had claimed that intelligence officers based at GCHQ had monitored the bombers' phone calls, but had failed or refused to pass information to RUC detectives hunting the killers in the days following the attack.
Following the programme last September, Gordon Brown commissioned Sir Peter to conduct a review of all intelligence material stemming from the bombing.
In his report, Sir Peter said: "I am satisfied that in the days surrounding 15th August and on the day itself, to the extent that any relevant intelligence was derived from interception, it was shared with RUC HQ and Special Branch South promptly and fully, and done so with the latter in accordance with procedures agreed with Special Branch South.
"No police witness before me was aware of any request to GCHQ being refused and there was warm praise from the head of Special Branch and the regional head of Special Branch South for the work done by GCHQ in Northern Ireland."
Sir Peter also said there was no evidence before him that police in the Republic had warned the RUC of a likely attack.
He also dismissed the programme's claims that intelligence officers had tracked the movements of the bombers' car, saying technology was not advanced enough in 1998 to do that.
He said: "The portrayal in the Panorama programme of the tracking on a screen of the movement of two cars, a scout car and a car carrying a bomb, by reference to two "blobs" moving on a road map has no correspondence whatever with what intercepting agencies were able to do or did on 15 August 1998.
"On the basis of evidence from an independent expert witness from a mobile communications service provider I am satisfied that in 1998, it was neither possible to track mobile phones in real time nor to visualise the location and movement of mobile phones in the way that was shown in the Panorama programme."
Michael Gallagher, whose son Aiden was one of the 29 people killed in the atrocity, said he did not accept that the Real IRA attack could not have been prevented.
"I think there was a large amount of high-grade intelligence available," he said.
''The fact that GCHQ themselves were monitoring - live or otherwise - demonstrates that.
"We had 27 attacks before Omagh, we had numerous amounts of warnings and one in particular to Omagh police station."
The chief constable of the Police Service of Northern Ireland, Sir Hugh Orde, in a letter to the NI Secretary of State Shaun Woodward, said: "I agree with Sir Peter's conclusions that such information, as was available to other agencies, was being passed to Special Branch promptly in line with agree procedures.
"I also strongly endorse his view that none of that information could have prevented the atrocity."
Mr Woodward, speaking in the House of Commons on Wednesday, said: "I commend Sir Peter Gibson for the thorough and exhaustive way that he has approached the task in looking at lessons to be learned in the sharing of intercept material on the day and around the time of the Omagh bombing."