There was unease about a suspected IRA mole based in an Garda station when two RUC officers were ambushed by the terror group, an inquiry has heard.
Superintendent Bob Buchanan was murdered along with colleague Chief Superintendent Harry Breen near the border after a meeting at Dundalk Garda Station in March 1989.
Retired detective superintendent Tom Connolly told the Smithwick Tribunal he “kept an open mind” when carrying out an internal probe into the killing in Dundalk.
The tribunal, established in 2005, is investigating allegations that Garda officers in the Republic or a civilian in the force colluded with the IRA in the murders.
Mr Connolly agreed there was unease over newspaper reports there was a mole in the station.
“I’d say I was aware of it possibly before I went to Dundalk. Certainly when I arrived in Dundalk I was told by some sources there was an unease about a certain individual,” said Mr Connolly, adding it had also been known outside the station.
The witness was not asked to identify this individual. Instead Justin Dillon, senior counsel for the tribunal, replied: “We will leave that for another day.”
It is understood Mr Connolly – who worked across Kildare and Dublin before being promoted to border superintendent in Dundalk in October 1998 – will return as a witness in the autumn.
The tribunal heard nothing had been highlighted by Mr Connolly’s investigation.
He said he believed the IRA would not have needed inside assistance to ambush the high-ranking RUC officers and that the pair could have been under surveillance for “months, possibly years”.
“I always have an open mind about everything, a concern about who could have been involved,” he added.
“But you asked me did they need any information and I said I do not believe they needed information.”
Mr Breen and Mr Buchanan were two of the highest-ranking RUC officers killed in the Troubles.
They had travelled to Dundalk Garda Station in Co Louth to discuss a possible joint RUC/Garda police operation against smuggling and were returning to Northern Ireland when they were ambushed just north of the border on the Edenappa Road.
Judge Peter Smithwick, who is heading the inquiry, had earlier launched an astonishing attack on the Republic of Ireland's justice minister for allegedly interfering with the tribunal.
The judge said witnesses were already reconsidering their co-operation with the inquiry since hearing about the Government-imposed time-frame, including one important witness from outside the State whose co-operation took some time to secure.
Justice Minister Alan Shatter announced last month the tribunal – which was set up in May 2005 but only began hearing public testimony this month – would have to finish its work by the end of November.
But in a statement last night he said it was made clear to the Oireachtas that if Judge Smithwick had difficulty in meeting the deadline, the Tribunal could report it and request an extension.
Mr Shatter said the tribunal has cost €8m to date.