DUBLIN — Detectives investigating the biggest mass killing committed by British troops in Northern Ireland, the Bloody Sunday massacre in 1972, have arrested on suspicion of murder one of the soldiers who opened fire that day — the first detention of its kind following decades of demands for justice.

A special police unit probing killings from the Northern Ireland conflict, the Legacy Investigation Branch, said it was interrogating the 66-year-old former member of the Parachute Regiment, the elite force that shot to death 13 Catholic protesters in the Bogside district of Londonderry. Catholic leaders and lawyers representing relatives of the dead welcomed the arrest and said they expected more retired soldiers to be arrested.

The detained man was identified as a former soldier who had testified, with his identity concealed, to two fact-finding investigations about his role in shooting at protesters when he was a 23-year-old lance corporal. To the fury of Irish nationalists, the original British probe in 1972 exonerated him and the other soldiers.

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But after 12 years, the most expensive fact-finding commission in British history concluded in 2010 that the soldiers had opened fire first and without warning, not in response to attacks from the outlawed Irish Republican Army; all but one of the Bloody Sunday victims were unarmed at the time they were killed; and none posed a threat to the soldiers.

British soldiers take cover while dispersing protesters with tear gas in Londonderry on Jan. 30, 1972, which became known as Bloody Sunday. PA / AP file

That British government-empowered commission also found that the former lance corporal and other soldiers had lied under oath when claiming to have targeted figures holding guns and grenades. Prime Minister David Cameron accepted the findings and in a parliamentary speech apologized to the families of Bloody Sunday victims, noting that the probe had found that the soldiers "knowingly put forward false accounts to seek to justify their firing."

Defence Secretary Gavin Williamson says "we are indebted to those soldiers who served with courage and distinction to bring peace to Northern Ireland" and says Government will offer "full legal and pastoral support to the individual affected by today's decision" — Sky News Breaking (@SkyNewsBreak) March 14, 2019

Bloody Sunday was a threshold event in Northern Ireland's conflict, driving radicalized Catholics into the ranks of the outlawed IRA and its campaign to force Northern Ireland out of the United Kingdom. Britain in response dissolved the province's Protestant-dominated government and imposed direct control from London as 1972 became the deadliest year of the entire four-decade conflict.

The former lance corporal is suspected of firing the shots that killed up to three of Bloody Sunday's dead: dockworker William Nash, 19, who was shot in the chest; and shop clerk John Young, 17, and bartender Michael McDaid, 20, who both were shot in the head.

A mural depicting the 14 people who lost their lives on Bloody Sunday in Londonderry, Northern Ireland. Charles McQuillan / Getty Images

Peter Madden, a lawyer who represents the families of Young and McDaid, said that any Bloody Sunday soldiers charged with murder or other crimes "must face public trial and will lose their anonymity."

If charged and convicted, the Bloody Sunday soldiers would be eligible for accelerated paroles under terms of Northern Ireland's 1998 peace accord, which allowed several hundred members of the IRA and other paramilitary groups from the British Protestant side of the community to walk free from prison within two years.

The families of victims of the events of Bloody Sunday say they are "disappointed" by the decision not to prosecute more than one former British soldier — Sky News Breaking (@SkyNewsBreak) March 14, 2019

Northern Ireland today is relatively peaceful. Paramilitary groups have stuck to mid-1990s cease-fires, the peace accord paved the way for a Catholic-Protestant unity government, and the IRA renounced violence and disarmed in 2005. British troops no longer patrol streets in support of the police but do continue to be based in Northern Ireland, reflecting its United Kingdom membership. IRA splinter groups opposed to the peace settlement still mount occasional gun and bomb attacks.