New evidence on the Omagh bombing has emerged to strengthen calls for an international inquiry into the atrocity, bereaved families have said.
The 1998 car bomb planted by dissident republicans opposed to the peace process killed 29 people in the Co Tyrone town, including a woman pregnant with twins.
The loss of life was the worst of any single attack of the Troubles and campaigners have demanded a cross-border inquiry into whether the authorities could have done more to prevent it.
Michael Gallagher, whose 21-year-old son Aiden was murdered in the attack, said a report being compiled at the request of the relatives has uncovered new information.
"We are confident this report will contain new evidence to support our call for a cross-border inquiry," he said.
"It is a work in progress. It is very difficult to put a time span on when it will be completed. There is much more material then we first thought."
He declined to comment on the nature of the new evidence, but predicted the report could be ready as early as mid-March and would add pressure on the governments.
Mr Gallagher's comments came as members of the Northern Ireland Affairs Committee visited Omagh and met some of the bereaved families.
Committee chairman Laurence Robertson laid a wreath on behalf of the MPs at a memorial to the victims of the Real IRA attack.
Mr Gallagher said he informed the committee of the report the Omagh Support and Self Help Group has commissioned from a group of London-based consultants.
He said the consultants have legal and investigative expertise and have started to trawl the huge amount of material already compiled on the bombing.
This includes police reports from both sides of the Irish border, a probe carried out by the Police Ombudsman in Northern Ireland, plus a government-sponsored review of how intelligence on the attack was handled.
Mr Gallagher said the relatives had also secured disclosure of other documents, while the consultants are interviewing individuals associated with the case.
He said the consultants were only three months into the work, but had already uncovered new information on the case.
"We are quietly confident that this is a report that the British and Irish governments will want to address," said Mr Gallagher.
"It is an uphill struggle but we feel that such is the evidence, that should the governments ignore this, we will then go to the courts."
Mr Gallagher said the consultants were not receiving any government assistance, but he added that the families planned to eventually approach police chiefs in Northern Ireland and the Republic of Ireland to offer them the chance to co-operate.
Four men have been held responsible by a civil court for the bombing. A man accused of murder was cleared by a court in 2007.
A 2001 report by former Police Ombudsman Dame Nuala O'Loan criticised poor judgment in the Royal Ulster Constabulary in the years after the attack.
Victims have repeatedly called for a cross-border public inquiry into alleged failings by police in the Republic of Ireland and Northern Ireland.