A 59 year old man has been arrested in Newry in Connection with the 1976 Kingsmills Massacre.
10 protestant workmen were killed in rural South Armagh after being ambushed by an IRA gang.
The suspect is being questioned on suspicion of ten counts of murder, and one of attempted murder.
The latest development comes just 2 months after police reopened the investigation into the case - haulting a long awaited inquest.
Detectives revealed a potential match had been made between a palm print found at the scene, and one already on police records.
Pensioner Alan Black survived the attack, despite being shot 18 times.
He's long called for justice in the case and was a leading campaigner in gaining the latest inquest into the killings.
Detectives from the PSNI’s Legacy Investigation Branch say the man is assisting with their enquiries.
They've also taken the chance to re-issue an appeal for information surrounding the murders.
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