A range of intelligence material could have been drawn together before the Omagh bombing in a bid to stop the killers in their tracks, the High Court in Belfast has heard.
A judge was told MI5 and RUC officers may have been been able to combine information that would have prevented the atrocity.
The claim came as the father of one of the victims continued his legal bid to force the Government to hold a public inquiry into the Real IRA attack.
Michael Gallagher’s son Aiden was among 29 people, including a woman pregnant with twins, killed in the outrage on August 15th, 1998.
In September 2013, Northern Ireland Secretary Theresa Villiers rejected calls for a public inquiry, deciding instead that a then-ongoing Police Ombudsman investigation was the best way to address any outstanding issues.
Last October, Northern Ireland Police Ombudsman Dr Michael Maguire published a report which found RUC Special Branch withheld some intelligence information from detectives hunting the bombers.
No one has ever been convicted of carrying out the attack.
Seamus Daly, a 44-year-old bricklayer from Cullaville, Co Monaghan, is currently charged with the 29 murders . He denies the charges.
Pressing ahead with the bid to have Ms Villiers’ decision judicially reviewed, Mr Gallagher’s barrister argued that neither the latest Police Ombudsman’s findings nor the charges against Mr Daly detracts from his challenge.
Article 2 of the European Convention on Human Rights includes a duty for state authorities to investigate fatal incidents.
Ashley Underwood QC told the court: “Our case here is that the bomb was at least arguably preventable.
“If it was arguably preventable then there needs to be an Article 2 investigation.”
At an earlier hearing it was contended there was a real prospect tracking equipment had been placed in the bomb car. Claims were also advanced that the terrorists behind a series of attacks leading up to Omagh were being monitored.
Mr Underwood on Wednesday referred to different strands of alleged material from GCHQ, MI5 and the RUC to back his submissions. “It’s the monitoring of the bombers as they crossed the border in the bomb car and scout car,” he said. “Our concern is the... strands of intelligence which could have all been put together on August 15th and alerted the authorities so as to stop the bombers in their tracks.
“There’s no reason to believe that has been investigated adequately or at all by any group of investigators.”
Paul McLaughlin, responding for the Secretary of State, stressed that there have already been four separate Police Ombudsman examinations linked to Omagh.
In the latest report, Dr Maguire concluded nothing had been identified which could have prevented the atrocity, but that Special Branch had acted “cautiously”, the court heard.
Mr McLaughlin argued that the legal challenge centred on the possibility of unknown gaps in the investigations.
Suggesting it may prejudice any criminal trial, Mr McLaughlin also contended that the information being sought - if it even exists - could emerge during those proceedings. He questioned whether the Article 2 obligation applied given the Human Rights Act which came into force in 2000 — two years on from the Omagh bombing.
Following submissions, Mr Justice Treacy confirmed he will rule next week on whether to grant leave to seek a judicial review.