A leading forensic expert has told the Smithwick Tribunal he was "furious" when he discovered that the scene of the Warrenpoint bomb attack in 1979 had not been preserved.
18 British soldiers died in the attack on 27 August 1979.
Dr Alan Hall first inspected the "nest" where the IRA unit had waited on the evening of Thursday 30 August, four days after the deadly attack.
But when he returned with a forensic team the following morning, he found "the complete area had been obliterated."
"I can't see what advantage you would gain by scything the vegetation to the ground," Dr Hall told the Smithwick tribunal.
"I was furious at the loss of potential evidence," he said. "I was furious that having gone to the effort of setting up a whole team to do a job that was no longer necessary."
Dr Hall said that the clearance was the result of "either unbelievable incompetence or deliberate obstruction."
Dr Hall said he had told the plainclothes garda in charge of the site the first time he visited it that he would return the following morning with a full forensic team.
He said he did not know the name of the officer he had spoken to.
A former RUC officer identified as Witness 68 told the tribunal on Wednesday that the plainclothes officer was Detective Sergeant Owen Corrigan.
Mr Corrigan, which is represented at the tribunal, denies the allegations of collusion which the tribunal is investigating.
Questioned by Mr Neil Rafferty, Dr Hall said he could think of no reason why a forensic site would be cleared overnight.
Dr Hall said his recall was that there were still items at the scene such as sandwich wrappers, from which fingerprints might be recovered when he first visited it.
Mr Dermot McGuinness SC on behalf of the garda commissioner said that the scene had already been forensically examined by garda forensic officers before Dr Hall arrived at the scene.
Documentary records showed that samples taken from various scenes by Garda Patrick Ennis on 28 August, two days before Dr Hall visited the site, were sent to the Garda Forensic Science Laboratory in the Phoenix Park.
RUC officer warned over security leaks
RUC special branch officers and a uniformed garda sergeant warned an RUC chief inspector assigned to the border in 1979 to be careful of what he said in the presence of garda officers because of security leaks, the Smithwick Tribunal heard earlier.
The retired officer, identified as Witness 73, was assigned to Bessbrook in 1979 to investigate the murder of four RUC officers.
He was one of the first investigators on the scene following the Warrenpoint ambush in which 18 British soldiers died in August 1979.
The detective chief inspector said he was told by RUC Special Branch to be careful what he said in front of garda detective sergeant Owen Corrigan.
The tribunal is investigating allegations of garda collusion in the IRA ambush which killed two RUC officers in March 1989. Mr Corrigan denies allegations of collusion, describing them as a "monstrous lie".
"It was suggested that he was passing on information to the IRA at the time," Witness 73 said.
He said he raised the issue with the divisional RUC commander in Newry, and with the operational head of CID at RUC HQ.
He said he was later told by a uniformed garda sergeant to be careful what he said because of leaks, but the garda sergeant did not name any particular officer.
The witness said he also worked during his career with garda officers in Donegal, and got on extremely well with them. He said he was "taken aback" and "shocked" when he was told the news.
Barrister for Owen Corrigan, Darren Lehane, said there was another man called Owen Corrigan, who was not a garda but who had been collated in the presence of senior IRA figures.
The barrister put it to the witness that he was reporting hearsay to the tribunal.
The witness said there was a difference between a briefing from a Special Branch officer and "tittle tattle and gossip."
He told the barrister he could not remember if the garda sergeant he spoke to was Dan Prenty. Mr Lehane said that other witnesses said Mr Prenty, who was a sergeant at the time, had "bad mouthed" Mr Corrigan to them.
