A Donegal publican was duped into making a statement admitting the murder of Raphoe cattle dealer Richie Barron in 1996, the tribunal heard.
Mr Barron's death was redesignated as a hit-and-run in 2002, the Garda Commissioner confirmed last year.
The inquiry was told the alleged confession, which Frank McBrearty jnr denied signing on December 4th, 1996, was written on a single side of a sheet of paper. "My theory is that a possibility was written up for him and he was duped," said former garda superintendent Kevin Lennon.
"The first page was written out," said Mr Lennon. "The next page, about his father not intimidating witnesses and all that, was put to him as a position. He never saw the first page."
"So that he signed the innocuous part?" asked Mr Justice Frederick Morris.
"Absolutely, sir," said Mr Lennon.
Mr Lennon said Det Sgt Jim Leheny later told him to "watch that statement. There was one-upmanship at play and the Dublin boys got caught. He said watch that statement - it was going around all day."
Mr Lennon said when he asked Sgt Leheny to report this in writing, the sergeant said: "You must think I'm mad. I'll deny that wherever I go."
The language of the statement did not match what a Raphoe man would use, said Mr Lennon, who was formerly in charge of the Letterkenny district. And he said the "arrogance" of Mr McBrearty jnr in earlier interview notes contrasted with the language of the alleged confession.
He said if he had taken the statement he would have asked many more questions to obtain a more detailed confession. And he added that the assault described in the statement did not match medical evidence about Mr Barron's injuries.
"So the facts of the statement do not jell with the known reality of the known facts," said tribunal barrister Paul McDermott.
"Who do you say is responsible for that?"
"The people who extracted that statement of confession are responsible," said Mr Lennon.
Solicitor Tom Murphy and Ms Sean Quinn said their clients, Det Garda John Fitzpatrick and Det Sgt John Melody of the NBCI, denied any allegations of wrongdoing in taking Mr McBrearty's statement. The chairman said he felt it appropriate that these denials should be reported in the press along with any allegations.
Mr McDermott asked the former superintendent why he had not reported his suspicions to senior members of the Garda. Mr Lennon replied that gardaí were "like a family".
"They work in sections, and some of those sections, if you take those sections on, you won't get away with it." He said if Mr McBrearty had been brought to trial, he would "have shouted it from the rooftops", but it was not easy for a "whistleblower" to report concerns in the force.