Mr Justice Nial Fennelly points to examples of conflicting and inconsistent evidence throughout his 300-page report.
But it is a phone call that took place on the afternoon of March 21 that presented the commission with one of its "most difficult and challenging issues".
Just days before the dramatic events surrounding Martin Callinan unfolded, Alan Shatter took a call from Michael Flahive - an Assistant Secretary in the Department of Justice.
Mr Flahive performed a senior role in the department and had oversight of the garda division.
Mr Flahive informed him for the first time of the garda tapes issue.
He also told the commission that he relayed to Mr Shatter the concern of the Attorney General Máire Whelan. But it was the manner in which the message was delivered that caused a dilemma for the Fennelly Commission.
Mr Shatter, who had just returned from a trip to Mexico, said he did not recall the phone call but accepted it may have happened. Significantly, though, Mr Shatter insisted he was not briefed about the garda tapes or the AG's concerns.
"Mr Shatter was in no doubt that, if he had known about the Attorney General's concerns, he would have had a conversation with her," the report said.
Mr Fennelly said he was taking "great care" with his consideration of the matter "since the credibility of each of these persons is in issue". He concluded that the "most likely explanation" is that the call took place but that Mr Shatter's "tiredness" following his trip may have impacted on him.
"The commission considers the most likely explanation to be that Mr Flahive did tell the Minister of the telephone recording issue but not in any urgent manner and that it did not register with the Minister due to tiredness at the end of a day following an overnight flight," the report said. Mr Flahive said he became aware of the emergence of the garda tapes on March 10.
This was a full 11 days before the minster in charge of his department. He had been asked by the Director General of the AG's Office, Liam O'Daly, to ensure Mr Shatter was informed.
But the Commission found that he "mistakenly" believed Mr Shatter had been appraised of the issue through a letter sent by gardaí to the Secretary General of the Department, Brian Purcell, on March 10. The letter was not passed onto Mr Shatter for over a fortnight. "Nonetheless, Mr Flahive innocently compounded the misfortune by assuring Mr O'Daly that the Minister was fully briefed."