Analysts have expressed doubts over Societe Generale's declaration that a single rogue trader was responsible for the fraud that cost it 4.9 billion euros ($7.1 billion).
"If you know the control procedures very well, then it is possible to elude them for a few days, maybe a few weeks. But it's hard to believe that he did this for a year," economist Elie Cohen told France-Info radio Friday.
If the man identified as 31-year-old Jerome Kerviel did manage it alone, it represents "an enormous breakdown" by France's second-largest bank, Cohen said.
The website of the daily Le Figaro reported Friday that, according to material in the hands of the public prosecutor of the Paris suburb of Nanterre, Kerviel allegedly began his scam in February 2007 and worked it until the middle of January 2008.
The bank has filed complaints against Kerviel, who began working for Societe Generale in 2000, for bank forgery and the use of forged documents.
Earlier Friday, the head of France's central bank, the Banque de France, Christian Noyer, said that an investigation into the affair would determine if Kerviel had acted alone and how he had managed to dupe the bank's internal controls without accomplices.
Many analysts in France and abroad were sceptical that a single trader, no matter how clever, could have carried out such a complex scheme for such a, long period of time without an accomplice or the tacit agreement of the bank's management.
Some analysts have suggested that the bank gave Kerviel a free hand in the hope he would be able to make up for losses it suffered because of the US subprime crisis.
On Thursday, Societe Generale also said that it would write down 2.05 billion euros in the fourth quarter of 2007 due to its exposure in the US.
According to Cohen, Kerviel built up positions of some 50 billion euros in trying to cover a series of losses he had suffered.
The Paris-based International Herald Tribune reported Friday that the fraud was not detected until last weekend, when auditors in the bank's risk management office noticed a series of fictitious trades on its books.
Societe Generale then closed out its exposure from Kerviel's trading in a market made volatile by the decision of the US Federal Reserve to cut its benchmark interest rates by three-quarters of a percent.
Kerviel was immediately dismissed, as were four of his superiors in Societe Generale's equities and derivatives division.
On Thursday, Noyer and Societe Generale head Daniel Bouton said they had no idea of Kerviel's whereabouts. But an attorney for Kerviel has told several French media that her client was not on the run and was waiting for formal notification of the charges against him.
The affair has moved several financial institutions to downgrade Societe Generale's shares, the daily Le Monde reported. Germany's Deutsche Bank has changed its recommendation from "buy" to "hold", while the American bank Bear Stearns said that the losses could lead to takeover attempts by competitors.
On Thursday, following revelation of the scam, Societe Generale shares lost more than four percent of their value. On Friday, following a volatile day of trading, the stock lost another 2.56 percent, finishing at 73.87 euros.
Since Jan 1, Societe Generale shares have lost more than one-fourth of their value.