AIB said preliminary investigations had revealed the suspected fraud at Allfirst in Baltimore had cost its treasury division $750m.
The FBI has now been called in to investigate the fraud and all foreign exchange operations at the US subsidiary have been discontinued.
The investigation will centre on the activities of one trader who failed to turn up to work on Monday after the US management first uncovered the fraud.
AIB is audited by PricewaterhouseCoopers, who declined to make any comment.
However, AIB’s finance director, Gary Kennedy, said AIB was not pointing the finger at the auditors, according to reports.
Since the fraud was discovered, a number of senior executives at Allfirst have been suspended pending completion of the investigation.
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