It took AIB five years to realise that instead of making millions for its US subsidiary, Allfirst, Mr John Rusnak had incurred losses of $691 million (€736 million) and entangled the Baltimore operation in the world's fourth-biggest financial scandal.
He came to international attention on February 6th when AIB reported it had been duped by a rogue trader who, the bank suggested, had gone missing.
AIB chief executive Mr Michael Buckley, described Mr Rusnak as a particularly "devious and clever" individual who had gone to extreme lengths to conceal the scale of his losses.
Initial investigations showed the church-going family man's trading activities were bizarre, and that even rudimentary checks of his trades and positions would have set alarm bells ringing throughout Allfirst. Far from devious, he managed to hide his mounting losses by creating fake documents on his personal computer, which went unnoticed by his supervisors.
A native of Pennsylvania, Mr Rusnak (37) joined Allfirst (then called First Maryland Bank) in 1993. He was pleased to leave the pressures of Wall Street and his post as a currency trader at Chemical Bank for Allfirst's smaller treasury operation.
Mr Rusnak portrayed himself as a trader with the ability to use very sophisticated options in complex trades and appears to have bamboozled his colleagues for much of his career in Baltimore.
His trading was pretty straightforward as he primarily made bets on how foreign exchange rates would move. He mostly focused on making money by correctly predicting the weakening of the Japanese yen against the US dollar.
He seems to have begun to incur losses in 1997 when he arranged to buy a large amount of yen at an agreed price sometime in the future. He had expected the Japanese currency to appreciate, allowing him to make a killing. Instead, the yen weakened and Mr Rusnak was left nursing a substantial loss.
The report into the fraud, compiled by US banker Mr Eugene Ludwig on behalf of AIB, suggests he continued to create fictitious option contracts that would appear to negate any losses over the next five years.
Between 1997 and 1999 he lost $90 million. In 2000, despite his best efforts to trade his way out of this nightmare, Mr Rusnak lost a further $211 million. The following year this climbed to $373.3 million and had risen to $691 million by the time his activities were finally uncovered in February 2002.
As part of Mr Rusnak's efforts to recoup his losses, he became a major trader on international markets and was openly courted by big financial institutions anxious to win his business. In Asian banks he was a near legendary figure, known as "Johnny Ruz", and executed his foreign exchange business predominantly through Citibank and Bank of America.
In the past couple of years, close colleagues had noticed a change in his demeanour with reports of bullying and aggressive behaviour towards more junior staff. He was also drinking heavily but no-one seems to have had any inkling about what was troubling this typical "Mr Middle America".
A closer examination of two of his fake documents in late January 2002 finally resulted in Mr Rusnak being rumbled. It subsequently transpired that Mr Rusnak was never a fugitive.
Last night, AIB welcomed the indictment of Mr Rusnak. "AIB stated at the outset that it believed it was the victim of a complex and sophisticated fraud and these charges endorse that conclusion," it said in a statement.
