Relatives of victims killed in Northern Ireland's deadliest single attack launched a civil action on Monday against five people they believe were behind the 1998 Omagh bombing.
Families of those killed had reacted with outrage in December when a judge criticised the police investigation into the attack and acquitted a man of murdering 29 people, meaning no one has been convicted nearly a decade later.
The only person so far jailed in connection with Omagh, bar owner Colm Murphy from the Irish Republic, had his conviction quashed by a Dublin court in 2005. He faces a retrial.
Lord Daniel Brennan, representing six families of those killed, described the attack as a "massacre of the innocents".
"Private citizens are confronting terrorism in our courts," he told Belfast's High Court in an opening statement.
Lawyers for the four men who are defending themselves against the claim objected to them being referred to as terrorists, describing it as prejudicial, but Judge Declan Morgan said opening remarks were not a statement of fact.
In a sign of how much things have changed since the bombing, the case will also move south to the Republic of Ireland in order to hear evidence from members of the Irish police.
A spokesman for Ireland's courts service said that while cases from other jurisdictions had sat in Irish courts before, it would probably be the first transfer from Northern Ireland.
The case, expected to run for up to eight weeks, begins as Northern Ireland this week marks the 10th anniversary of a peace agreement which largely ended 30 years of violence that killed more than 3,600 people.
But tension persists between minority Catholics, who mostly want to see the province reunited with the rest of Ireland, and majority Protestants seeking to preserve British sovereignty.
The Omagh bombing, four months after the 1998 peace deal, wounded more than 200 people and was carried out by the Real Irish Republican Army, a breakaway faction opposed to a 1997 IRA truce in its campaign to oust Britain from Northern Ireland.
"The previous court appearances have always been acts carried out by the state and not always necessarily on our behalf," Michael Gallagher, whose 21-year-old son Aidan was killed in the bombing, told Reuters.
"This is something that we have complete control of and that is important."
