PARIS — Judges trying French futures trader Jerome Kerviel probed Wednesday how he could have overstepped his authority so completely as to wind up gambling tens of billions of euros (dollars) of Societe Generale's money.

When his trial opened Tuesday, Kerviel argued that he never hid what he was doing from his bosses, who sat close to him in the trading room. The scandal cost euro5 billion (more than $7 billion at the time) in losses once the bank unwound Kerviel's positions in January 2008.

Kerviel's lawyers argue that his superiors tacitly encouraged his massive risk-taking as long as he made money. The bank denies that claim, insisting that Kerviel methodically concealed his tracks.

The 33-year-old trader is not accused profiting personally from his huge trades, and he insists he only wanted to make money for the bank.

Kerviel's defense team also paints him as a pawn of an out-of-control banking system that encouraged massive risk-taking in the pursuit of ever-greater profits — a system that has since imploded. His case gave a taste of the much larger banking crises to come, from the fall of Lehman Brothers to Bernard L. Madoff's multibillion-dollar Ponzi scheme.

Societe Generale stunned the banking world when it accused Kerviel of massive fraud in 2008. It said Kerviel overstepped his authority and bet 50 billion euros ($73 billion) — more than the bank's market value — on futures in European equity markets. The risk limit for Kerviel's entire 8-member trading team was only euro125 million.

Asked by the head judge if he was mandated to bet tens of billions, Kerviel responded "Probably not." Then he amended his response: "Certainly not."

Kerviel is accused of concealing his actions by balancing out his real trades with fictitious transactions. The bank spent days quietly unwinding his positions before it announced the scandal to the markets.

Kerviel said Societe Generale traders often overstepped their limits, adding, "we were never given a hard time" by managers.

The former chairman of the French stock market regulator, Jean-Francois Lepetit, testified Wednesday that traders did occasionally go over their limits but were obligated to inform their supervisors immediately.

"There is no justification for remaining silent," he said. "If you go over your limits and you don't say it, you are at fault."

Questions remain about how the bank missed so many warning signals about Kerviel. An internal report by the bank said managers failed to follow up on 74 different alarms about Kerviel's activities.

The former futures trader faces a possible five years in prison as well as a euro375,000 ($450,000) fine if convicted on charges of forgery, breach of trust and unauthorized computer use.

The bank, a civil party, is also asking for nearly euro5 billion in damages — the amount lost in the scandal — even though Kerviel could never pay that amount.