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 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.
Earlier, a retired RUC chief inspector said he was warned by RUC special branch officers and a uniformed garda sergeant to be careful what he said in the presence of garda officers because of security leaks.
The retired officer, identified as Witness 73, said he was told by RUC Special Branch to be careful what he said in front of garda detective sergeant Owen Corrigan.
“It was suggested that he was passing on information to the IRA at the time,” he said.
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.
Barrister for Owen Corrigan Mr 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 Mr 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.