The Smithwick tribunal, which is investigating alleged collusion in the IRA murder of two senior RUC men, has been given the name of a garda who allegedly set them up.

The garda's name is contained in an internal RUC Special Branch document on the murder of Chief Superintendent Harry Breen and Superintendent Bob Buchanan 20 years ago. The document was given to the tribunal by a former senior RUC officer, a source said.

It is one of several significant documents drawn up during the RUC investigation into the double murder. The name of the garda, who has since retired, is known to the Sunday Tribune.

Breen and Buchanan were shot dead in south Armagh as they made their way home from a meeting at Dundalk garda station. They were the most senior police officers to be killed in the Troubles.

Former RUC officers have alleged that the IRA was tipped off about the men's journey and given details of their movements by a garda who worked for the Provisionals.

The tribunal, chaired by retired judge Peter Smithwick, was set up by the government four years ago to investigate collusion claims.

An ex- RUC officer told the Sunday Tribune that the meeting on 20 March 1989 was arranged three days in advance, which would have given the IRA ample time to plan the attack after a tip-off from a garda informant.

Breen had told colleagues he "didn't like the set-up", believing that the same garda named in the document given to the Smithwick tribunal worked for the IRA.

Breen attended the meeting reluctantly. The IRA's strength in south Armagh meant it was too dangerous for the RUC men to have a British army accompaniment.

An IRA informer has claimed that he and M, a senior IRA figure at the time, travelled to Dundalk while Breen and Buchanan were inside the garda station. He claimed that a garda left the building and told the two IRA men that the RUC officers would be leaving shortly.

A six-strong IRA team was waiting for the policemen as they drove along the Edenappa Road, near the village of Jonesborough. Two gunmen in combat gear and balaclavas fired into their car.

Buchanan tried to reverse but the vehicle stalled. He died strapped in the seat. Although hit, Breen managed to get out of the car and wave a white handkerchief. He was shot in the back of the head.