Relatives of the Omagh bomb victims are seeking talks with the head of MI5.
They have asked Dame Eliza Manningham-Buller for a meeting in a bid to obtain new details of the intelligence agency's role in exposing Michael McKevitt, leader of the Real IRA which carried out the atrocity.
FBI agent David Rupert, an American, and a key witness in the 2003 Dublin trial in which McKevitt, 55, was convicted of directing terrorism at the time of Omagh, also worked inside the dissident republican terror organisation for MI5.
Michael Gallagher whose son Aiden was killed in the August 1998 bombing said: "Many of the court documents confirm that MI5 played a role, no matter how peripheral in the investigation. We already know Rupert was one of their agents and its important we try and find our more."
The Americans paid Rupert £1.25m. for infiltrating the Real IRA and getting close to McKevitt from Dundalk, Co.Louth. Last month McKevitt failed in his appeal to have his conviction overturned after being sentenced to 20 years.
Rupert is also likely to be called as a witness when a High Court civil action is heard against the five men they claim were responsible for the attack which left 29 people dead.
But that case will not go ahead until after the trial, expected in Belfast later this year of Sean Hoey, 36, of Molly Road, Jonesborough, south Armagh who has been accused of the 29 murders.
The Omagh relatives are having talks in Dublin tomorrow with the Garda Commissioner Noel Conroy and in Belfast on Friday with the head of the Northern Ireland Human Rights Commission Monica McWilliams who is being urged to back their campaign for a full cross-border judicial inquiry.