Fresh evidence that police could have intercepted the Omagh bombers before they committed the atrocity has been uncovered in a new independent inquiry, victims' relatives have claimed.
Bereaved families who commissioned the report said it has identified "hard proof" that officers on both sides of the Irish border were involved in a live operation on the day of the attack.
Twenty-nine people, including a woman pregnant with twins, were killed when the Real IRA bomb ripped through the Co Tyrone market town on August 15 1998.
The family campaigners have previously demanded a cross-border inquiry into whether the authorities in Northern Ireland and in the Republic of Ireland could have done more to prevent the bombing.
Relatives who commissioned the report met Northern Ireland Secretary Owen Paterson in Belfast ahead of presenting the final version to the UK Government next month.
Michael Gallagher, whose 21-year-old son Aiden was murdered in the attack, said the meeting was "hot and heavy".
"We told him we have evidence that there was a live police operation going on on August 15 that could have intercepted the bombers," he said. "This is new evidence that hasn't been made public before."
He added: "We also have evidence that public officials lied to the general public about the Omagh bombing. We will be producing all of that evidence to him on June 18."
Mr Gallagher said he would not be outlining full details of the evidence unearthed until the report had been passed to the Government.
The Omagh Support and Self Help Group commissioned the report from a group of London-based consultants who have interviewed key figures associated with the case. The consultants have also examined material already compiled on the bombing.