Relatives of the victims of the 1998 Omagh bombing met Gordon Brown at Downing Street today to ask for a cross-border public inquiry into the police investigation of the atrocity.
29 people, including a mother pregnant with twins, died in the Real-IRA car bombing.
No one has been convicted for the attack.
The families also asked the prime minister to make intelligence material relating to the investigation available to the Belfast judge who is hearing a civil case against five suspects.
Michael Gallagher, who lost his son in the bombing, said the families had been told by Gordon Brown that he believed there was no further information in his possession that could lead to a further criminal prosecution.
BBC's Panorama programme claimed the British intelligence listening station GCHQ had intercepted information that could have prevented the bombing but that it was not passed on to the police.
A British government inquiry last month rejected the BBC claim but the families want the GCHQ intelligence material made available to the judge in their civil action.
Today's meeting was also attended by PSNI Chief Constable Sir Hugh Orde, Northern Ireland Secretary Shaun Woodward and Business Secretary Lord Mandleson who is a supporter of the civil action being taken by the families against those suspected of involvement in the bombing.
