Five FBI agents are assisting the Garda in the investigation into the shooting of Mr John Carthy in Abbeylara, the Minister for Justice confirmed yesterday.
Mr O'Donoghue said the investigation report would be published by the end of the month and that 200 people had been interviewed.
The Minister said he spoke recently to the Garda Commissioner, Mr Pat Byrne, about the need to secure outside expertise.
"Five FBI agents, whose expertise in the handling of the type of situation which arose in Abbeylara would be internationally recognised, are here and are assisting in the matter," he said.
It is understood the agents arrived in Ireland earlier this week.
Mr O'Donoghue stressed that it was not part of the role of the agents to conduct the investigation. "That is a matter for the investigating chief superintendent," he said.
"What they will do, however, is assess the handling of the operation as a whole by reference to international best practice."
The Minister said he expected the input of the FBI would be reflected in the chief superintendent's report. "The primary purpose is to ensure that any lessons that may need to be learned from this tragic death will be learned," Mr O'Donoghue added.
The Minister told the Fine Gael spokesman on Justice, Mr Jim Higgins, that it was not practice for the Commissioner to disclose the operational guidelines on the use of firearms. Stun guns were not issued or used by gardai, he said.
Mr O'Donoghue told Mr Higgins that what happened to Mr Carthy raised important issues of public interest.
"I intend to make public the findings of that investigation. I ask that people be patient in the interim, and await the outcome of the investigation. We will decide then where to go from there."
Members of the Garda unit involved in the killing of Mr Carthy have trained with the FBI at its headquarters at Quantico in Virginia, writes Jim Cusack, Security Editor.
The FBI instructors will be looking at the whole Garda operation from the point when the Emergency Response Unit was called to Longford to Mr Carthy's death the following day.
There are well-laid guidelines for handling armed sieges. Most of the concerns surrounding Abbeylara are about the level of firepower deployed by the Garda and the way in which the armed officers were controlled.
The gardai were close to the house and were armed with assault rifles, machineguns and handguns.
The gardai also allowed television pictures to be broadcast of the scene, showing officers in camouflage fatigues and wearing body armour. It is felt that images of the siege and commentary on television may have agitated Mr Carthy.
The chief superintendent investigating the incident earlier this week interviewed officers from the Garda Press Office about the position of media and their contact with the force during the incident.