A man has been arrested by detectives investigating the sectarian murder of 10 Protestant workmen in the North 40 years ago.
The 59-year-old suspect was detained two months after police announced a major forensic breakthrough in the unsolved investigation into the mass shooting at the village of Kingsmill, Co Armagh.
At the end of May, the Police Service of Northern Ireland (PSNI) revealed a potential match had been found to a palm print left on a getaway vehicle used by the republican paramilitaries.
The IRA was widely blamed for what was one of the most notorious killings of the Troubles.
The factory workers were ambushed as they travelled along the Whitecross to Bessbrook road in rural south Armagh on January 5, 1976 in an attack seen as a reprisal for loyalist killings in the same area.
The men's minibus was stopped by a man waving a red light and those on board were asked their religion by a camouflaged gunman with an English accent, whom the victims thought was a soldier.
The only Catholic workman was ordered to run away.
The killers, who had been hidden in the hedges, ordered the rest to line up outside the van and then opened fire.
The getaway vehicle used by the gunmen was left abandoned across the Irish border. The palm print was discovered later.
It was re-examined by forensic scientists only days after a long-delayed inquest into the deaths got under way in Belfast.
A lawyer for the PSNI told stunned relatives attending the hearing that a potential match on the police's database had been made.
One man survived the attack, despite being shot 18 times.
The suspect was arrested in Newry, Co Down on the suspicion of the ten murders and one attempted murder.
Alan Black, the sole survivor of the gun attack, said news of the arrest had come like a "bolt from the blue".
He said: "I don't really know what to think to be honest.
"I am still trying to take it in.
"For 40 years the police were not interested in Kingsmill but this, I suppose, is a development at least.
"We will just have to wait and see what happens and whether any charges are brought against this person."
Colin Worton, whose brother Kenneth was killed in the 1976 massacre, said: "We were disillusioned when we were told at the inquest about the discovery of the palm print after 40 years.
"But I believe this has to be a positive development.
"We will have to wait and see the justice system take its course."
Karen Armstrong, who lost her brother John McConville, said: "We will have to wait and see how it all pans out."
Stormont Assembly member Danny Kennedy, a prominent supporter of the Kingsmill families' quest for justice, described the arrest as a "potentially positive development".
"The Kingsmills massacre in 1976 shocked not only Northern Ireland but much farther afield," said the Ulster Unionist representative.
"The sectarian massacre of 10 Protestant workmen by the IRA remains a stain on south Armagh.
"The families and sole survivor have waited 40 years in their pursuit of maximum truth and justice for this brutal and barbaric crime.
"The news that someone has been arrested in connection with Kingsmills must be seen as a potentially positive development. We must now wait and allow the police investigation to take its course."