The validity of a crucial witness statement was not questioned by any garda at a pre-arrest conference during the investigation into the death of Raphoe cattle dealer Mr Richie Barron, the Morris tribunal heard.
The statement by Mr Noel McBride, which placed Mr Frank McBrearty jnr and Mr Mark McConnell coming from the area where Mr Barron was found at a crucial time, was later withdrawn, and Mr McBride said he had been "put up to it" by a police informer.
However, Supt John McGinley said the statement was taken "at face value" at the time. "This was a young man who presented himself without any agenda, we believed," he said. "Put together with all the factors before it, it was decided there was enough there to arrest Frank McBrearty and Mark McConnell," he added.
The superintendent said it was also decided to arrest other people "who were giving them alibis and so on".
Supt McGinley said that parts of the statement where Mr McBride referred to other people who the guards knew were in Raphoe that night, made them believe it was genuine.
"Did anybody actually identify the elements in the statement that made it look genuine?" asked chairman Mr Justice Morris.
"Yes chairman," said the superintendent. He said he did not think anyone had warned about the statement at the conference.
"In our view it seemed to amount to a measure of corroboration at the time," said Supt McGinley.
"We didn't think that the man had any agenda. And we had no reason to think that there was any underlying thing going on there."
"Are you saying now that was wrong?" asked tribunal barrister Mr Peter Charleton.
"Looking back on it now, what we did clearly wasn't adequate. However, where we were at the time we genuinely believed that there was sufficient there, and it was supported by other factors there," he said.
"We genuinely thought that we were on the right road, and we thought that for a considerable time," he said. "When one looks back on it now, one can have a different view."
The superintendent said the investigation team "would not have gone down that road" if they had believed there was anything wrong with the statement.