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."