Exchange officials warned Société Générale about Jerome Kerviel's deals late last year, a Paris prosecutor said.
Jerome Kerviel
After examining preliminary evidence, a public prosecutor said Mr Kerviel (31) admitted hiding his activities from superiors but had said other traders also played fast and loose with bank rules.
Prosecutor Jean-Claude Marin also said Eurex, a derivatives exchange owned by Deutsche Boerse, questioned Mr Kerviel's trading positions in November, but that the former back office worker had been able to fool his employer.
"Eurex alerted Société Générale in November 2007 about the positions taken by Jerome Kerviel. Questioned by the bank, he produced a fake document to justify the risk cover," Mr Marin said.
Eurex said its processes and controls "functioned correctly at all levels, also in this case".
SocGen has said it was in the dark about Kerviel's alleged illicit trades until it spotted a discrepancy on January 18th, triggering an internal investigation.
France's second-biggest bank was also hit by lawsuits over a director's share sale that was made before the biggest rogue trading scandal in history broke, and over the way it unwound Mr Kerviel's €50 billion ($74 billion) of bets into last week's sliding markets at a cost of €4.9 billion.
In a new political twist to the scandal, Bank of France chief Christian Noyer revealed that he delayed telling the government about it for several days because he feared a leak.
Mr Kerviel told police he concealed his trades because he wanted to enhance his reputation as a trader, not out of any desire to hurt the bank, prosecutor Mr Marin told a news conference.
Mr Kerviel's lawyer said: "He has not embezzled anyone, he hasn't taken a cent for himself and he was just doing his job as best he could."