The families of the victims of the Omagh bombing say they will take legal action against the British and Irish governments if its agreed not to hold a full cross-border independent inquiry.
In a specially commissioned report by the families, it is claimed there were significant investigative and intelligence failures on both sides of the border in the lead up to and in the aftermath of the atrocity.
The families handed the report to the governments last June, but say they have yet to receive a substantive response.
Michael Gallagher, whose son Aidan died in the bombing, said the lack of answers from the governments was "prolonging the agony of the families".
Any future court action would come in the form of judicial review proceedings.
Amnesty International has joined the families in calling for a full, independent cross-border investigation into the 1998 bombing.
The Real IRA attack in the Co Tyrone town claimed the lives of 29 people, including a woman who was pregnant with twins.
Next week marks the 15th anniversary of the bombing and pressure on the British government to agree to an inquiry is likely to intensify.
Relatives today only published a small part of the report collated for them by London based security experts, insisting the remainder was too sensitive for public consumption.
One of the most significant sections, the families claims, are files of 4,000 emails detailing communication between an FBI agent, who had infiltrated the Real IRA at the time of the bombing, and his handlers.
While emails from agent David Rupert, who was apparently working in conjunction with MI5, have already featured heavily in past court proceedings, the families claim that a number which indicated that a bombing was planned have never been made public.
Relatives contend that the messages identify Omagh as a potential target and establish a time frame consistent with the eventual attack.
Amnesty's Northern Ireland programme director Patrick Corrigan has said the families bereaved and those injured by the bomb are still left seeking the full truth, and wondering if it could have been prevented.
Mr Corrigan said questions remain outstanding about alleged state failures in the lead-up to and the aftermath of the bomb.
In particular, he said, there are unanswered questions concerning the gathering and sharing of intelligence material between domestic and international agencies, including the RUC, MI5, the FBI and gardaí.
The families were critical of the stance taken by Taoiseach Enda Kenny and Minister for Justice Alan Shatter to their campaign.
A spokeswoman for the Department of Justice said Mr Shatter is still considering the report, presented to him by the group in July 2012.
Former Northern Ireland police ombudsman Dame Nuala O'Loan, who while in office carried out her own investigation into the bombing, and former Metropolitan Police assistant commissioner and counter terrorism chief Bob Quick today publicly backed the call for an inquiry.
She said what had emerged in the 15 years since the attack was "cause for enormous concern".
Northern Ireland Secretary Theresa Villiers was briefed last February and the report is also with the Spanish government. Two of the dead and 11 of the 200 injured were from Spain.
Several men have faced charges in connection with the attack, but nobody has ever been convicted of the murders.