LONDON — Nearly 25 years after a car bomb killed 29 people in the Northern Ireland town of Omagh — the deadliest single attack of the era known as the Troubles — the British government said on Thursday that it would open an independent inquiry into whether security forces could have averted the bombing.
The decision, which came after a court ruled in 2021 that there was evidence that the bombing could have been prevented, is a striking victory for families of the victims, which had campaigned for a new inquiry for more than a decade.
It comes at a delicate moment when the government is advancing so-called legacy legislation, which would grant immunity from prosecution to those who cooperate in investigations of unsolved killings from the three decades of the Troubles. Those inquiries would be conducted by an independent commission.
Though Omagh is part of that bloodstained history, it falls into its own tragic category. The bombing, in August 1998, occurred four months after the signing of the Good Friday Agreement, which is credited with ending the sectarian violence of the Troubles. It was viewed as a last spasm of terror by a splinter group of the Irish Republican Army, the Real I.R.A., which fiercely opposed the peace accord.
Britain’s decision also comes at a pivotal moment in negotiations between Britain and the European Union over post-Brexit trade arrangements for Northern Ireland. Prime Minister Rishi Sunak has signaled he would like to strike a deal before the 25th anniversary of the Good Friday Agreement in April, which could draw President Biden and others to Belfast to celebrate its achievements.
“Having carefully considered the judgment of the High Court,” Britain’s Northern Ireland secretary, Chris Heaton-Harris, said in Parliament, “I believe that an independent statutory inquiry is the most appropriate form of further investigation to address the grounds identified by the court.”
“The Omagh bomb was a horrific terrorist atrocity committed by the Real I.R.A., which caused untold damage to the families of those who were tragically killed and injured,” Mr. Heaton-Harris declared in his statement. “Its impact was felt not just in Northern Ireland, but across the world.”
Among those killed in the midafternoon attack in Omagh, a busy market town, were a woman pregnant with twins, two Spanish tourists, six teenagers and six children. Nobody was convicted of the attack in criminal a court, but four members of the Real I.R.A. were found “liable” for it in a civil case in 2009.
The independent inquiry, which will be chaired by an as-yet-unnamed senior judge, will investigate four issues identified by the court: how the authorities handled and shared intelligence, how they analyzed cellphone data, whether there was advance knowledge about the plot and whether the authorities could or should have conducted an operation to disrupt the Real I.R.A.’s attack.
Questions about the bombing have festered for decades. British, Irish, and American security agencies were accused of withholding intelligence about the Real I.R.A. from the Royal Ulster Constabulary, the police in Northern Ireland. In 2008, the BBC reported that GCHQ, Britain’s electronic surveillance agency, eavesdropped on cellphone conversations between the bombers on the day of the attack.
“It is a big risk for the government to take,” said Monica McWilliams, an academic and former politician who was involved in the 1998 peace negotiations. Security agencies, she said, might be compelled to disclose sensitive national security information.
British officials estimated that the inquiry would take at least two years and could stretch out much longer. Ms. McWilliams said its effectiveness would depend on its powers and on the credibility of the person who chairs it.
A hastily conducted inquiry into the Bloody Sunday massacre in Derry in 1972, which largely exonerated British troops, was discredited as a whitewash. A subsequent, more thorough investigation, conducted by Mark Saville, a former justice of the British Supreme Court, found that soldiers had fired on fleeing unarmed civilians.
While political analysts said there was no direct connection between the inquiry and the trade talks between London and Brussels, they said the announcement could influence the mood in Northern Ireland at a critical time.
Katy Hayward, a professor of politics at Queen’s University in Belfast, said that this context was “very much in people’s minds in the U.K. government on anything Northern Ireland-related at the moment.”
The British government has played down a recent media report that it was on the brink of a deal with the European Union. “Substantial gaps” between the two sides remain, a Downing Street spokesman said on Wednesday.
Probing the secrets of Omagh, Ms. McWilliams said, could contribute to a deepening sense of reconciliation between nationalists and unionists in the North as the anniversary of the Good Friday Agreement approaches.
“We can’t bring anybody back from the dead,” she said. “But it’s a very timely announcement, given that there’s so much angst surrounding the legacy legislation.”