It blamed the killing on a failure in its chain of command, saying those involved had been acting on instructions but the IRA leadership had not sanctioned the operation.
The postal worker, Frank Kerr, 53, was fatally wounded by a shot fired as he struggled with raiders in the main postal sorting office in Newry. Two men appeared in court last week charged with his murder.
The incident led to concerns that the cessation of military operations announced by the IRA at the end of August was in doubt. It provoked angry reactions across the political spectrum, with many calls for all paramilitary groups to hand in their arms.
Last night's statement will provide some assurance that the ceasefire is not at immediate risk, though the incident still represents a considerable blow to the IRA's reputation for internal discipline.
The initial republican reactions to the incident were confused, with the IRA announcing it was launching an investigation. It first issued a statement asserting the cessation of military operations still held, but it did not at first confirm or deny that it had sanctioned the robbery attempt.
Last night, having completed its investigation, it said: ``Those carrying out the robbery were `acting on instructions' but the so-called operation had not been sanctioned by the IRA leadership.'' Responsibility for the incident lay with ``an identified problem in the IRA chain of command'' which, it said, had now been rectified. It added: ``We have established that Frank Kerr was shot in the midst of an intense scuffle with one of our volunteers and we take this opportunity to offer sincere apologies to his family and friends.''
The question remains of how such a fundamental misunderstanding of IRA orders could have been made by individuals in the organisation. Since the ceasefire there have been IRA ``punishment beatings'' but no other use of arms, and Sinn Fein spokesmen have spelt out that the cessation should be taking as meaning an end to the use of weaponry. Martin McGuinness, for example, said after the robbery: ``I take the IRA at their word. I think complete means complete. That's my opinion of the IRA statement.''
The latest IRA statement now suggests that a figure in their organisation senior enough to give instructions had not grasped this point.
Suspicions are bound to linger that not everyone in the ranks is entirely happy with the cessation, but the IRA affirmation that it remains committed to the peace process, together with the now clear understanding that armed robberies are not permissible, will come as a relief to most observers.
In particular, there will be relief within the British and Irish governments, particularly since the former is due to open exploratory talks with Sinn Fein within the next few weeks. Sinn Fein has said Mr McGuinness will lead the republican team in those talks.
After the robbery, the Northern Ireland Secretary, Sir Patrick Mayhew, called on the IRA to issue an outright condemnation of the incident. He said then he hoped it would not affect the timetable for the talks with Sinn Fein.
Within hours of the killing of Mr Kerr the Irish government announced that it was halting its plans to allow early releases of nine republican prisoners.
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