The Irish and British governments have said they need time to consider the implications of a landmark court ruling into the Omagh bombing.
Mr Justice Horner said there is a real prospect the 1998 Real IRA bombing could have been prevented by the security services.
He called for new investigations on both sides of the Irish border.
Taoiseach Micheál Martin said the judgment demanded "very serious reflection and analysis" by both the UK and Irish governments.
"It's those who perpetrated the crime itself, who thought up the idea, who planted the bomb and left such devastation behind them, we can never lose sight of that, they're fundamentally guilty in terms of murdering so many people," he said.
"But the state must always self-reflect in terms of how it acts to protect its citizens."
Mr Martin highlighted that the Irish Government had previously fulfilled its obligations in respect of investigating crimes with a cross-border dimension when it set up the Smithwick Tribunal to probe allegations of Garda collusion in the IRA killings of two senior RUC officers during the Troubles.
"We will do so again in respect of any further investigations that we support and cooperate with," he said.
"But I think we have to take one step at a time, we have to analyse what the implications of this are, reflect on the judge's conclusions and how we can take it forward from here."
Secretary of State Brandon Lewis said the British government is considering the judge's statement.
“I want to put on record my deep regret that the families of those killed and wounded have had to wait so long to find out what happened on that terrible day in 1998," he said.
"They deserve answers and I have great respect for their patience, grace and determination.
“We recognise that today the Court has set out that there are ‘plausible allegations that there was a real prospect of preventing the Omagh bombing’ and that more should be done to investigate this.
“The UK Government will take time to consider the judge’s statement and all its recommendations carefully as we wait for the full judgment to be published."
Sinn Fein West Tyrone MP Orfhlaith Begley said the verdict will raise further questions for the Omagh families.
"That is why it is vital that the British Government act on the judge's recommendation and immediately move to announce a full, human rights-compliant investigation into the exact circumstances of what happened," she said.
SDLP West Tyrone MLA Daniel McCrossan said there must now be a full public inquiry into the bombing.
"There can be no more excuses, the truth must be heard and justice must be done," he said.
Ulster Unionist MLA and former victims' commissioner Mike Nesbitt said the full ruling by Mr Justice Horner will need to be scrutinised when it is released.
"Whatever detail is contained in the full ruling, the fact remains that the Omagh bomb was the work of the terrorists who chose to make it, transport it and detonate it in a market town on a busy August Saturday afternoon," he said.
Alliance councillor for the area Stephen Donnelly said: "Today is a major vindication for the families and their ongoing campaign pursuit for truth and justice. The bombing was the darkest hour in the history of Omagh and its shadow persists to this day."
