A retired Garda superintendent said he had been concerned that a top RUC officer repeatedly used the same car to travel to the Irish Republic in the months before he was killed in an IRA ambush.
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 Garda Superintendent Pat Tierney, based in the town at the time, told the Smithwick Tribunal in Dublin that he met Mr Buchanan 12 times in both Northern Ireland and the Republic between early February and the day he died.
"I certainly was a little apprehensive," Mr Tierney said. "I was conscious of the fact that he was coming quite often in the same car. It was a concern."
The tribunal, established in 2005, is investigating allegations that Garda officers in the Republic or a civilian working in the force colluded with the IRA in the murders.
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.
Mr Tierney, who was in charge of the Dundalk District office at the station, told the tribunal he had no evidence of any Garda or civilian collusion with the IRA.
Tribunal lawyer Dara Hayes said PSNI intelligence received more than 10 years after the killings claimed meetings between gardai and the RUC were organised by a civilian administrator based at an unknown location in Ireland.
But Mr Tierney and another witness, former Garda George Flynn who also worked in the district office at the time, said they had no knowledge of this.