The relatives demanded to know what the state was trying to hide after a parliamentary committee that conducted its own inquiry into the bombing revealed the Prime Minister had denied them access to the review of the security services' role.
The Northern Ireland Affairs Committee has called for a fresh investigation into the part played by intelligence agents in the yet unsolved dissident republican attack 12 years ago and if crucial data that could have led to arrests was kept from police officers.
But after outlining the inquiry findings at Stormont this morning, committee chair Sir Patrick Cormack criticised Gordon Brown for only letting them see a heavily edited version of the government's own report on the controversial claims carried out by Intelligence Services Commissioner Sir Peter Gibson.
Michael Gallagher, whose 21-year-old son Aiden was of the 29 people killed in the Real IRA attack, said this was not acceptable and reiterated the bereaved families' long-standing demand for a full public inquiry into the bombing.
"This (the committee) is the watchdog on government in Northern Ireland and here they are saying there are a number of very serious deficiencies, that the government is refusing to co-operate, the Prime Minister did not allow Sir Patrick to see the Gibson report in private, even in the House of Commons.
"I think the honest average man in the street will always say there must be a reason, other than the reason that it could give an advantage to terrorists.
"Sir Patrick wasn't going to give any advantage to a terrorist so there's not much left for families to believe other than that government have something there that's going to embarrass them and they don't want to disclose that to the families.
"That's the only possible answer that I can imagine."
Downing Street defended the decision not to release Sir Peter's report to the committee, highlighting that the chair of the UK Intelligence and Security Committee Kim Howells had seen it.
"Obviously, when national security is involved, there can only be a limited number of people with who that can be shared," said a spokesman for Mr Brown.
No one has ever been successfully convicted of the Omagh bomb, with the only man jailed in connection with the attack, 57-year-old Co Louth builder Colm Murphy, cleared last month after a retrial in Dublin.
However, last year four men, including Murphy, were found liable for the bombing in a landmark civil case taken by the victims' families.
While the affairs committee stopped short of calling for a public inquiry, Mr Gallagher said the truth of what happened on that day in August 1998 could only be found through such a judicial hearing.
"I think it is important that we only have one further inquiry and that should be an overarching inquiry that looks at what happened on both sides of the border and moves it forward," he said.
Godfrey Wilson, whose 15-year-old daughter Lorraine was killed, echoed his call.
"We're looking for a cross border public inquiry, a full judicial public inquiry," he said.
"There's too many questions on both sides of the border - before, during and after the tragedy - that haven't been answered.
"I feel it would help my life to know how the things progressed."
The committee undertook its inquiry into the security services' role following claims in a BBC documentary that the Government's listening station GCHQ had monitored suspects' mobile phone calls as they drove to Omagh from the Irish Republic on the day of the atrocity.
Panorama said this information was never passed to Royal Ulster Constabulary detectives assigned to the case.
The subsequent review by Sir Peter Gibson rejected many of Panorama's assertions.
However, only an edited version of the commissioner's report has ever been made public.
Committee chair Sir Patrick said he accepted Sir Peter's assurance there was nothing in the full version that contradicted the abridged format, but said the government's decision to deny him access would not assure public confidence.
"Sir Peter Gibson is a man who came and gave evidence to our committee, he himself made it quite plain that he would have no personal objection to my seeing his report," he said.
"He assured us that there was nothing in the unpublished version that would contradict the published version. The Prime Minister and the Secretary of State (Shaun Woodward) and the chair of security and intelligence committee (Kim Howells) have also given us that assurance.
"We are not in any way saying that these men aren't telling the truth, far from it, but what we are saying is seeing the full report would enable me to assure my colleagues in the committee and in turn assure a wider public that everything had been properly taken into account."
After reviewing the edited summary of the Gibson report, committee members agreed with the commissioner's claim that information obtained by GCHQ was not monitored in 'real time' and therefore could not have prevented the bombing.
But it raised concerns about the data flow after the attack, especially whether names of the suspected bombers were known and, if so, why they were not passed to police officers.
In particular the inquiry said there was a need to establish the part played by RUC Special Branch - the police's anti-terrorism unit - and whether it was handed data by GCHQ but failed to pass it on to RUC colleagues in the Crime Investigation Department (CID) who were working on the case.
As well as calling for a fresh examination of the intelligence, the committee's report also:
* Found that questions remain about whether the bombing could have been pre-empted by action against terrorists who carried out earlier bombings in 1998
* Called for a definitive statement on whether the names of those thought to have been involved in the bombing were known to the intelligence services, Special Branch, or the RUC in the days immediately after the bombing, and if so, why no arrests resulted
* Asked the Government to justify the argument that the public interest is best served by keeping telephone intercepts secret rather than using them to bring murderers to justice
* Called on the UK's Intelligence and Security Committee to reconsider how any intercept intelligence was or was not used
* Recommended that the Government considers providing legal aid for the victims of terrorism if they bring civil actions against suspected perpetrators once criminal investigation has failed to bring a prosecution.
Panorama claimed intelligence officers had tracked the movements of the bombers' car and a scout car on their way to Omagh.
However, in his review Sir Peter said technology was not advanced enough in 1998 to do that and insisted the vehicles were not being followed in 'real time', meaning the information could not have thwarted the bombing.
Sir Peter said information on the bombers taken from telephone intercepts examined in the wake of the event was passed to police. But he did not reveal whether this data included written transcripts of the phone calls.
He also said there was no evidence before him that police in the Republic had warned the RUC of a likely attack.
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