The bank said that trader Jerome Kerviel used computer passwords belonging to others as well as his experience of the controls placed on traders by the back office to make high-risk trades.
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Ralph Silva, an analyst at TowerGroup, said this is speculation.
"What we do know is he was very familiar with back-office operations, which would absolutely help him do this."
He said the bank, the back-office operation and trade clearing houses all had systems that should have highlighted Kerviel's activity sooner.
Kerviel, who is currently being questioned by police, can be held until this afternoon when he must either be released or handed preliminary charges.