Former Garda commissioner Pat Byrne accepted just months before he retired that the murder of Seamus Ludlow had not been properly investigated.
Responding by letter to the Barron inquiry in January 2003, Mr Byrne said that senior officers had not "grabbed this key issue by the neck" in the 1970s and 1980s.
In a 12-page letter, he wrote: "Where does the fault lie? In my view, it lies with all the senior management aware of the information of the 30th January, 1979 [the intelligence passed by the RUC to the Garda about four suspects for the murder of Seamus Ludlow]; people in decision-making positions in the public service, either here or in Northern Ireland; but the main responsibility must rest with, I'm sorry to say, the Garda Síochána, the investigating agency of this murder.
"Deputy commissioners, assistant commissioners, chief superintendents and superintendents were aware of this information.
"Those directly involved in the case and those with collective responsibility for overseeing crime investigation and extra-jurisdiction co-operation were aware and still this vital aspect of the investigation was not pursued.
"The only explanation I can give, looking back on the 70s and the 80s, is that with the pressure of work and the demands on the organisation, it was lost sight of and not pursued.
"In my view, this explanation is not adequate and is of little consolation to the relatives and friends of the late Seamus Ludlow.
"With the passage of time, the loss of memory of some, the questionable memory of others and the unavailability of certain people, I don't see how, in any forum, the questions you ask can be answered adequately," he said.
Earlier in the letter, he had stated: "Notwithstanding the demands on both police forces north and south of the Border during the relevant years, it is not acceptable that the basics of any serious crime investigation, particularly murder, were not, without comprehensive and fully supported reasoning, pursued to the limit.
"I accept that hindsight is of great benefit when analysing situations of yesteryear. Nevertheless, I am satisfied that Garda management in this case was somewhat remiss in doing its business.
"It appears that nobody grabbed this key issue by the neck and made a decision, committed to writing, to either pursue or abandon the investigation," he wrote.
Dealing with Supt John Courtney's claims that he had told Deputy Commissioner Lawrence Wren, Mr Byrne noted that Mr Wren had been transferred from his security and intelligence posting on December 11th, 1979.
Furthermore, Mr Byrne said it "seems strange" that "the paper trail runs out" when Supt. Courtney was ordered the following March by the head of C1, which probed "ordinary" crimes of the day, to report on developments in the Ludlow case.
"A response that Deputy Commissioner Wren had directed no further action would have finalised the matter for Supt Courtney," he went on.
It would have been "much simpler", he said, if Supt Courtney had sought directions from his own direct superiors, rather than going through C3, which was charged with dealing with subversives.
Though he said he had never experienced political interference during his time as Garda Commissioner, Mr Byrne said he believed that senior Department of Justice officials would have been informed in 1979 of the RUC's information about the suspected killers.
"Since 1969, when one considers the challenges of the time it was vital that on issues that affected the State, there should be at all times an open line of communication between the Garda Commissioner and his management team and the Minister, the secretary of the Department (of Justice) and his officials," wrote Mr Byrne.