A former British soldier is being questioned by detectives on suspicion of murdering three civil rights demonstrators in Londonderry on Bloody Sunday.
The 66-year-old ex-paratrooper, who was detained in Co Antrim on Tuesday morning, is the first person arrested by detectives probing the deaths of the 14 people killed after the Army opened fire on crowds of protesters in Derry's Bogside in January 1972.
It is understood the pensioner was arrested on suspicion of the murders of William Nash, 19, John Young, 17, and Michael McDaid, 20, all of whom were shot dead in close proximity to one another at a rubble barricade on Rossville Street.
It is believed the former soldier is also being questioned about the attempted murder of William Nash's father Alexander. Mr Nash came to the barrier to save his son but was shot in the arm and body.
It is understood the soldier gave evidence to the Government-commissioned inquiry into Bloody Sunday, undertaken by Lord Saville, under the cipher Lance Corporal J.
Kate Nash, William's sister, welcomed the development.
"We have always fought very hard to be treated equally within the justice system," she said.
"I see this as a positive step."
Thirteen people were killed by members of the Parachute Regiment on the day of the incident. Another victim of the shootings died in hospital four months later.
The Police Service of Northern Ireland's murder investigation into the events of Bloody Sunday was launched in 2012.
The probe was initiated after the Saville Inquiry found that none of victims was posing a threat to soldiers when they were shot.
Following the publication of the Saville report in 2010, Prime Minister David Cameron apologised for the Army's actions, branding them "unjustified and unjustifiable".
In September, the PSNI announced their intention to interview seven former soldiers about their involvement on the day.
The suspect detained on Tuesday by the PSNI's Legacy Investigations Branch has been taken to a police station in Belfast for questioning.
The officer leading the investigation, Detective Chief Inspector Ian Harrison, said the arrest marked a "new phase in the overall investigation".
He said the phase would continue for "some time".
Mickey McKinney, whose brother William McKinney was one of those killed, also welcomed the arrest.
"We are hopeful this is the start now of bringing in suspects to be questioned," he said.
"Our quest for justice goes on. We are not going to stop until the people responsible for the murders are in court and sentenced."
Lawyers representing the Nash, Young and McDaid families welcomed the development in the investigation.
However, Democratic Unionist East Londonderry MP Gregory Campbell asked if all killings around the time of Bloody Sunday would be investigated.
"It remains to be seen whether the current investigations will focus solely on the actions of soldiers on that day or whether progress will be made on arresting others who were engaged in illegal terrorist activity at the same time," he said.
"Two police officers were in a patrol car on part of the route of the march three days before Bloody Sunday and were murdered by the Provisional IRA.
"The police need to confirm if they are questioning anyone in relation to that double murder or the other murders that occurred around the same time.
"Are they following any lines of inquiry against individuals who were not police or army personnel serving in Londonderry at that time?"
Sinn Fein Assembly member for Foyle, Raymond McCartney, said the ex-soldier's arrest was a "step forward".
"This is another step forward in the long campaign for justice by the Bloody Sunday families," he said.
He added: "I would call on the PSNI to ensure the relatives are kept up to date of all developments on the investigations."
A Government spokeswoman said it would not comment on an individual case.
She added: "So far as our overall approach to these matters is concerned, the Government believes in the rule of law. Where there is evidence of wrongdoing it is right that this should be investigated and, where the evidence exists, for prosecutions to follow.
"We remain unstinting in our admiration and support for the men and women of the police and Armed Forces whose sacrifice ensured that terrorism would never succeed in Northern Ireland, and that its future would only ever be determined by democracy and consent.
"The overwhelming majority carried out their duties with courage, professionalism and integrity. This Government will never forget the debt of gratitude we owe them."
Former Northern Ireland secretary Lord Mandelson told Channel 4 News: "There are perils in going back so far into history.
"Perils over the evidence that is available, people's memories, people's ability to produce their own evidence and facts so long ago to present to some sort of tribunal or commission or court of law or whatever it is.
"I think there's a broader point too which is that peace in Northern Ireland is quite fragile, community relations are fragile and we have to think very hard before we do anything that is going to make the fragile worse, which is going to fire up tensions between different parts of the community.
"I'm sure that a lot of thought has been put into this and I'm not going to second guess those who have made this decision, but I do hope that before further decisions of this kind are taken, people think long and hard about the perhaps unforeseen consequences of going so far back into the past."