A relatives' group of people killed and injured in the 1974 Dublin-Monaghan bombings have said they will take their case to the European Court of Human Rights.
Thirty-three people died on the Troubles' bloodiest day
Justice for the Forgotten claimed they now have evidence to back up claims of collusion in the loyalist bombings which killed 33 people.
A total of four bombs exploded and resulted in the biggest loss of life on a single day in the Troubles.
Solicitor Greg O'Neill said the group had "moved beyond the suspicion and speculation."
"Having spent 11 years working on this case in different forms, the families and their lawyers are satisfied we have now prima facie evidence of collusion and participation in the bombings," he said.
Survivors and relatives of those killed in the bombings want a public inquiry into the attacks, which has been ruled out by the Irish Prime Minister Bertie Ahern.
Mr Ahern has ruled out a public inquiry into the bombings
However he said his government remained committed to finding the truth about what happened in the bombings.
The Ulster Volunteer Force admitted 10 years ago that it was responsible for the bombings.
However, no-one has ever been convicted of the attacks which injured more than 250 people.
In April, an inquest into the bombings re-opened almost 30 years after the attacks.
The inquest came a month after an Irish parliamentary committee recommended that a public inquiry into the bombings should be held in the UK.
It also recommended an inquiry into the investigation by the Gardai at the time.
Last December, a report by Mr Justice Henry Barron said there were grounds for suspecting the bombers may have had help from members of British security forces, but there was no conclusive proof.
However, this did not rule out the involvement of individual members of the security forces.
Many of the grieving relatives believe the UVF was helped by British intelligence service operatives aiming to warn the Irish Government not to interfere in Northern Ireland's affairs.
The bombings took place while Protestant workers held a general strike in Northern Ireland to bring down the power-sharing government set up under the Sunningdale Agreement.