Police in the North say a potential suspect in the murder of 10 people there has been identified 40 years after the attack.
The deaths of textile factory workers outside the village of Kingsmill was widely blamed on the IRA.
A forensic re-examination of a palm print left on the getaway vehicle has led to the breakthrough.
Relatives of the victims have been informed.
The significant development in the long-unsolved investigation into the killing of the textile factory workers outside the Co Armagh village of Kingsmill in January 1976 has come only a week after a new inquest into the incident began.
The IRA was widely blamed for what was one of the most notorious outrages of the Troubles.
The getaway vehicle used by the killers was left abandoned in the Republic of Ireland. The palm print was discovered later.
It was re-examined by forensic scientists last week and a potential match on the police’s database was made.
Relatives of the victims were informed about the development ahead of the scheduled sitting of the inquest on Tuesday.
A lawyer for the PSNI later informed coroner Brian Sherrard, who described it as a “massive development” in the case.
Outside court, the sole survivor of the attack, Alan Black, who was shot 18 times, questioned the police’s timing.
“It’s nearly unbelievable that they had it all this time and now, when we’re into the inquest, they suddenly find a match,” he said.
“It’s unbelievable they couldn’t have done this before.”
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 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.
Assistant Chief Constable Mark Hamilton, head of the PSNI’s legacy and justice department, said: “Due to a recent forensic development, detectives from Legacy Investigation Branch are now following a line of inquiry in relation to the murders at Kingsmill in 1976.
“A senior investigating officer has now been appointed in order to take this forward.
“We have been liaising with the Coroner’s Office in relation to this and will continue to do so.
“The families have also been informed. I appreciate this is a very difficult time for them; however, we are committed to progressing this matter as expeditiously as possible and will keep them updated as appropriate.”
