A Northern Ireland court has said that the 1998 Omagh bombing could have been prevented by UK authorities.
Judges have ordered the UK and the Republic of Ireland to open an investigation into the car bombing that killed 29 people.
High Court Justice Mark Horner said the inquiries needed to determine whether a more "proactive" security approach could have thwarted the attack.
Another 220 were injured in the car bombing on 15 August 1998, which took place in a small shopping street in the Northern Irish town of Omagh.
The car bombing at Omagh was carried out by the group calling themselves the Real Irish Republican Army (Real IRA), but no one has been held criminally responsible for the attack.
Victims of the blast included a woman pregnant with twins, and many young people, including two Spanish tourists.
The bombing was the worst single atrocity of the Troubles, a three-decade-long conflict involving Irish republicans, British loyalist paramilitaries, and UK troops.
But judges on Friday found there were "plausible" claims that "there was a real possibility of preventing the Omagh bombing".
"I am satisfied that certain grounds when considered separately or together give rise to plausible allegations that there was a real prospect of preventing the Omagh bombing,'' Horner said at the court in Belfast.
"These grounds involve the consideration of terrorist activity on both sides of the border by prominent dissident terrorist republicans leading up to the Omagh bomb."
The case was brought eight years ago by Michael Gallagher, whose son Aiden was killed in the blast.
Gallagher had challenged the British government's refusal to conduct a public inquiry into the bombing.
He claimed that the bombing could have been prevented if British security agents and police officers from the Royal Ulster Constabulary had combined their intelligence on dissident republic groups.
Two anonymous calls had raised the alarm 40 minutes before the bomb went off, but police claim the location given for the attack was wrong.
The UK had argued that a police ombudsman investigation was the best way to address any outstanding issues in the Omagh attack. Northern Ireland Secretary Brandon Lewis said that the government would wait to review the full decision before deciding how to proceed.
"We recognise [what] the court has set out today ... and that more should be done to investigate this," Lewis said in a statement released after the ruling.
"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."
Judges in Belfast did note that they had no authority to compel officials in the Republic of Ireland to conduct their own inquiry, but said that a parallel inquiry across the border would be a "real advantage".
Irish Prime Minister Michael Martin has said that his government would analyse the judgment and do "what is necessary for the citizens of the island of Ireland".
Earlier this month, the UK government presented a controversial plan to end all prosecutions related to the Northern Ireland conflict, denounced by all sides as an "amnesty".