The Saville inquiry found testimony by Lance Corporal J and two other soldiers — who testified that they had opened fire on men they believed to be armed — to be “knowingly untrue.” The three soldiers were believed to be involved in the deaths of Michael McDaid, 20, William Nash, 19, and John Young, 17.

“We are sure that these soldiers fired either in the belief that no one in the areas toward which they respectively fired was posing a threat of causing death or serious injury, or not caring whether or not anyone there was posing such a threat,” the report found.

John Kelly, whose brother Michael, 17, was killed on Bloody Sunday, welcomed news of the arrest. “All of the families of the victims are very excited by it: this is the first and we expect all of the rest of those responsible to be brought in and prosecuted,” he said.

Marc Mulholland, an associate professor of modern history at Oxford, said the 2010 report was “universally seen as fair-handed and comprehensive,” but left open the legal and moral question of whether individuals should be held culpable.

“It is an episode that is universally known in Northern Ireland, and its resonance echoes down the years,” he said in a telephone interview. “So some people will wonder why it’s taken so long for someone to be held to account. On the other hand, there will be those who feel the arrest will reopen, yet again, an episode that should be closed by now. Lots of people have had to bear pain, see killers of friends and family walk the streets again after 1998. Should we be going down this path?”

Brian Conway, a sociologist at the Maynooth University in Ireland, said he was not surprised by news of the arrest, which he called “part of a wider attempt to heal the divisions of the past.”

The arrest was made by the Legacy Investigation Branch, a unit of the Police Service of Northern Ireland established last December to examine old cases. In a statement, the police quoted the officer leading the investigation, Detective Chief Inspector Ian Harrison, as saying that “today’s arrest marked a new phase in the overall investigation,” which he said could “continue for some time.”

Also on Tuesday, after an investigation by the same unit, a former British soldier, Dennis Hutchings, 73, was charged with the attempted murder of a mentally disabled 27-year-old man in the hamlet of Benburb in 1974. The man, John Pat Cunningham, was shot in the back as he ran from an army patrol. The British government apologized for his death in 2013.