"I don't know where he is," said Daniel Bouton, the long-standing chief executive and chairman of Société Générale, when he was asked yesterday about the whereabouts of Jérôme Kerviel, the trader at the centre of one of the biggest frauds in banking history.

But while Mr Kerviel's movements remained unknown, the sequence of events that led to the extraordinary writedown at SocGen was becoming clearer.

He joined the bank in 2000 and worked in Paris. The first three years were spent in the bank's "back office" and "middle office", where trades are settled and risk is managed. SocGen said he had never worked directly in its risk control section, but remained in contact with people in those areas so he could be updated with the bank's risk controls.

"The reasons he could succeed was because the trader knew intimately the bank's risk controls and swiftly shifted positions to evade detection at each level of control," Mr Bouton said.

By the end, it is alleged that Mr Kerviel was operating a hugely complex fraud. He was reportedly logging into the bank's computers under different identities and was reporting extremely complex, but fictitious, trades.

The fraud was discovered after the trader made an error with a fictitious counterparty. Its extent became clear over the weekend, when the bank's management interviewed Mr Kerviel.

SocGen bankers who talked to him last weekend about his escapade were struck by his unreal sense that he had discovered a smart new trading strategy that would bring the bank big profits.

Jean-Pierre Mustier, head of investment banking, who was among those at an interview with the trader at the weekend, said: "I am convinced he worked alone."

SocGen said Mr Kerviel was responsible for trading futures on European equity market indices, and had taken "massive fraudulent directional positions" in 2007 and 2008, many of which had made a profit in 2007. It was positions he had taken since the start of the year that caused the losses. SocGen said it had filed criminal charges against Mr Kerviel, who is now subject to the bank's dismissal procedure.

Asked why it had not revealed the fraud earlier, Mr Bouton said the bank could not have brought charges earlier because if it had done so, the scale of his exposure could have leaked, with serious consequences for the company and the wider market.

Mr Kerviel's annual salary - including bonus - was less than €100,000. "He has not yet received his bonus for 2007, but I don't think he will claim it," Mr Bouton said.