The witness, who was an RUC chief inspector in Newry in 1989, gave evidence from behind a screen to protect his identity, and was identified only as Witness 33.
The witness said he was told to “be careful who I talked to” in Dundalk garda station by Garda inspector Dan Prenty.
Another officer, Witness 27, a divisonal commander, also told him to be careful what he said, and named detective sergeant Owen Corrigan.
“He just said to be careful what you talk to him about. He didn’t elaborate.”
In evidence, Witness 27 did not express any concerns about Mr Corrigan, tribunal barrister Fintan Valentine BL said.
And Mr Jim O’Callaghan on behalf of Mr Corrigan said that Witness 27 had told the tribunal he trusted Mr Corrigan with his life.
“He didn’t specify he was any kind of threat or problem, just be careful what you say,” the witness said. “You could take all sorts of things out of that.”
The tribunal is looking at claims a Garda “mole” leaked information to the IRA leading to the murders of two RUC officers, Harry Breen and Bob Buchanan, as they returned from a meeting in Dundalk Garda station.
Mr Corrigan is one of three retired sergeants granted legal representation at the tribunal. He has described allegations that he leaked information at a “monstrous lie”.
The tribunal also heard that the road chief superintendent Breen and superintendent Buchanan travelled on was marked “out of bounds” until 11am on the day they were killed.
The witness said he last saw Mr Buchanan on the day he travelled south, and Mr Buchanan invited him to join him on the trip, but he declined because of a prior appointment.
After the attack, the witness was flown by helicopter to the ambush scene, and was the first RUC officer on the scene.
“I didn’t stay around too long, I was just down to identify the bodies,” he said. “Bearing in mind the scene hadn’t been cleared at that stage, I didn’t touch anything.”
The witness said there was speculation among RUC officers about the possibility of a Garda leak.
“Some people thought it had to be something internal inside, in other words from the Guards or civilians working there,” he explained.
He said that 20 to 30 people could have been involved in the ambush, and they could have been stationed on more than one road.
But he said he had no knowledge of “technical information” received by the RUC about the routes. Witness 27, told the tribunal last week that there was an increase in radio noise around midday on the day Breen and Buchanan died..
Later, former RUC divisional commander Brian Lally said that he knew Mr Corrigan, no no one ever told him that he should be careful around the Garda detective sergeant, or watch what he said in his presence.
He said that any security concerns about a Garda leak would be passed on to officers of equal rank in the force, but it would probably be done verbally rather than in writing for security reasons. He said he was in charge of uniformed officers, and so would not necessarily hear about intelligence passed between the detective branches.
Mr Lally, who was succeeded by Mr Breen at his retirement, said that gardai had told him “in the warmest and kindest terms” to speak to Mr Buchanan, saying “”For God’s sake would you talk to Bob and have him cool down, he’s in the station almost every day.”
Mr Lally said that face to face meetings were necessary in the job to build trust between officers, and Mt Buchanan was keen to do a good job.
Quizzed about the dangers of building patterns which could be identified by the IRA, he said there were “only so many ways” that some trips could be taken, although times could be varied too.
“There’s an acceptance that gets into your mind, I have a job to do and I’ll do it,” he said.
Thanked at the end of his evidence by tribunal chairman Mr Peter Smithwick, Mr Lally said “I came out of respect for the tribunal and in memory of my colleagues.”