It was likely Irish Garda (police) officers colluded in the IRA murders of two senior Northern Ireland police officers, an inquiry has found.

Chief Superintendent Harry Breen and Superintendent Bob Buchanan, the highest-ranking RUC officers to be killed during 30 years of The Troubles, were shot dead in an ambush in March 1989 in south Armagh.

The attack happened as they crossed the border into Northern Ireland after a meeting in Dundalk Garda station.

In the report of his Dublin-based eight-year inquiry released on Tuesday evening, Judge Peter Smithwick said he was “satisfied there was collusion in the murders.”

Smithwick said the circumstances suggested information was leaked to trigger the IRA operation, and the timing suggested it was “more likely that the information came from Dundalk Garda station.”

Irish Justice Minister Alan Shatter apologised “without reservation” for the failings identified in the report.

“Even with the passage of 24 years and the positive developments which have taken place on the island since, our condemnation of their murder should be as strong today as it was then,” he said.

A solicitor speaking on behalf of Breen’s family said the report was “a truly remarkable exposé and indictment of wrongdoing and collusion with terrorists by some within An Garda Siochána.”

The family said the report detailed in the most stark and dramatic fashion the failure by state systems to address these matters year upon year.

Bob Buchanan's son, William, said, “The findings of Judge Smithwick are both incredible and shocking, and confirm the existence of a mole in Dundalk station. This led to my father’s death.”

Smithwick said there was not enough weight of evidence to make a determination that key tribunal witnesses, former Sergeant Leo Colton, or former Detective Sergeant Owen Corrigan, colluded in the murder of the RUC men.

The report ruled out the possibility of a third key witness, Sergeant Finbarr Hickey, having colluded in the murders.  All three men repeatedly denied passing information to the IRA.

In the years following the killings Finbarr Hickey served a jail sentence for signing blank application forms for passports which ended up in the hands of members of the IRA. Hickey said he had done so at the behest of Colton, who repeatedly denied this.

Corrigan, who acknowledged dealing in second hand cars and owning a number of properties while being a serving officer in Dundalk, said he became disillusioned with the force and eventually left. He maintained throughout that the allegations against him were “a monstrous lie.”

He was named in the House of Parliament by MP Jeffrey Donaldson as the mole who passed information to the IRA which led to the ambush and murder of the two RUC officers.

He won damages in a High Court case when he was named in a newspaper article as an IRA mole.

Garda Commissioner Martin Callinan, who received the report of several hundred pages on Tuesday evening, said, “Given the serious matters under examination by the tribunal, the report, conclusions and recommendations will now need to be carefully examined. It would be inappropriate to comment further at this stage.”

Garda Commissioner Martin Callinan responding to The Smithwick Tribunal: