(The Wall Street Journal) -- Société Générale's account of how a rogue trader triggered billions of euros in losses is coming under strain, as it emerges that his risky trades may have begun over a year earlier than the bank has said and warnings were repeatedly missed.

Jérôme Kerviel, whom the bank has portrayed as an ingenious fraudster, sometimes used simple tricks to cover his tracks, according to investigators on the case. One of them was to fabricate email messages from nonexistent trading partners to deflect supervisors' concerns about unusual trades, a police official said.