Relatives urged the London and Dublin governments to tell the whole truth about alleged security gaffes in the lead-up to the Real IRA attack in August 1998, and the subsequent investigation, as they presented parts of a new report documenting their claims.
The families say the information, which they are only publishing in part because they claim most is too sensitive, outlines new evidence that indicates opportunities were missed to prevent the bombing and subsequently to catch the killers.
Twenty-nine people, including a woman pregnant with twins, died when the car bomb ripped through the Co Tyrone town.
While no one has been criminally convicted of the crime, four republicans were found liable for the atrocity in a landmark civil case taken by some of the bereaved relatives and ordered to pay £1.6 million compensation.
Michael Gallagher, whose son Aidan died in the bombing, said: "We feel there was an enormous amount of intelligence available, that intelligence was not used properly and as a result of this we have had no convictions.
"It's very damning and it's a blot on both governments. For that reason we are calling on both governments to come together and tell the truth, tell what happened on that day."
The two governments were given the families' report in June last year. Mr Gallagher said the relatives have yet to receive a substantive response from either.