It has emerged that the weapon used by the Provisional IRA in the ambush of RUC Chief Supt Harry Breen and Supt Bob Buchanan, whose murders are the subject of the Smithwick Tribunal, had been used in several other murders, including the Kingsmills massacre.
Ten Protestant workers were killed in January 1976 when they were taken from a minibus and shot by the IRA near the village of Kingsmills in Armagh.
One of the other murders was that of Eamon Maguire. His body was found at Cullaville, south Armagh in September 1987. The IRA said they killed him because he was an informer.
His death has been linked to the British agent known as Stakeknife who was involved with the IRA's internal security unit.
The man who denies he is Stakeknife, Freddie Scappaticci, has legal representation at the Smithwick Tribunal.
The information about the gun formed part of the investigation by the North's Historical Enquiries Team into the Kingsmills murders.
The information has been passed onto the Smithwick Tribunal.
The gun, a .223 calibre AR-15 Armalite, was also responsible for the murders of Lance Corporal Philip James and Private Roy Bedford, both members of the Parachute Regiment, who were killed in an ambush by the IRA in Crossmaglen, south Armagh on March 16th, 1974.
Catholic contractor, Terence McKeever was murdered by the IRA in June, 1986 in Cullyhanna, south Armagh using the same gun because he was carrying out electrical work for the British army.
Chief Supt Breen and Supt Buchanan were the last two people to be murdered using the gun.
The weapon was found by the RUC during a search in Crossmaglen in April 1990. During its 16 years in use it was also used in several attempted murders in the south Armagh area.
The tribunal also heard that one of the RUC officers murdered in an IRA ambush took no security precautions.
Former Garda Inspector Vincent Rowan told the tribunal that he was a sergeant on duty in Dundalk Garda Station on the day of the ambush in which Supt Buchanan and Chief Supt Breen were killed.
The two RUC men had left a meeting at the station and had just driven across the border when they were ambushed and killed by the IRA.
The tribunal is investigating claims that a garda colluded with the paramilitaries in the murder of the two men - the most senior RUC officers to die in the Troubles.
Mr Rowan said that he had brought tea into the meeting between them and the Garda Chief Supt John Nolan.
Afterwards he spoke to Supt Buchanan when he was leaning on the banister.
It was a regular thing for him to stay around chatting. He said he had congratulated the Superintendent on his promotion.
When news of the shooting came back he said he recognised the registration number of the car involved as the vehicle belonging to Supt Buchanan.
He said the Superintendent regularly visited the station, parking his car out the front. He took no security precautions, said Mr Rowan.
He said he had no knowledge of a mole being in Dundalk garda station.
