Two senior republicans are seeking to go before the European Court of Human Rights in a bid to overturn a ruling that they were liable for the Omagh bombing.
Lawyers for Michael McKevitt and Liam Campbell have based the challenge on their inability to cross-examine an FBI spy whose evidence was central to the case against them.
They are also contesting the decision to allow hearsay evidence from David Rupert in the case.
The disclosure comes as two other men originally held responsible, Colm Murphy and Seamus Daly, prepare to face a retrial at the High Court in Belfast.
No-one has been convicted of the bomb attack that devastated the County Tyrone market town in August 1998.
It subsequently emerged that the device was tracked from its source to its destination by British military intelligence and RUC police Special Branch without being intercepted.
Two telephoned warnings failed to clear the area around the vehicle which carried the bomb, resulting in the greatest single loss of life throughout the conflict.
McKevitt, Campbell, Murphy and Daly were all held liable for the bombing in a civil ruling in 2009.
Mr Justice Morgan, who is now the North’s ‘Lord Chief Justice’, ordered them to pay 1.6 million pounds in compensation.
The Court of Appeal subsequently upheld Murphy and Daly’s challenges to the verdict and ordered them to face a retrial which gets underway next week.
However, McKevitt and Campbell failed to overturn the rulings against them.
A further petition to the Supreme Court in London was also rejected, leaving them with one final option of seeking to go to Europe.
Legal papers prepared on their behalf focus on the role and credibility of Rupert, an American trucking boss-turned FBI spy who infiltrated the ‘Real IRA’ in return for cash.
He is on a witness protection programme after testifying for the prosecution at the criminal trial of Mr McKevitt in Dublin in 2000.
Although Rupert was forbidden from attending the original civil action, emails between the spy and his handlers were submitted as evidence.
The lawyer representing McKevitt and Campbell have objected to that move.
Kevin Winters confirmed: “Having exhausted all domestic court processes both of my clients have lodged applications with the European Court of Human Rights.
“The key areas that the court will be asked to look at is the reliance on the hearsay evidence of David Rupert, and the inability to cross-examine him about his claims.”
Papers lodged on behalf of the pair will undergo a preliminary assessment of the points raised.
It is expected that the European Court of Human Rights will make a provisional ruling on the merits of each application by July.