PARIS — Cloaked in a black robe, the French judge gazed down at the accused rogue trader who nearly brought down Societe Generale bank and asked commandingly: "Who are you, Monsieur Kerviel? Answer the question."

As the trial opened Tuesday, that question is the mystery at the heart of the scandal: Why would a soft-spoken young man from the provinces with an apparently ordinary life have gambled tens of billions of his bank's money — even more than its market value? Judges pored over Jerome Kerviel's psychological evaluation, his performance reviews and even his long-ago internships to try to find out.

The scandal led to euro5 billion (more than $7 billion at the time) in losses once the bank unwound Jerome Kerviel's positions in January 2008. The case gave a taste of the spiraling crises to come in the finance world, from the fall of Lehman Brothers to Bernard L. Madoff's multibillion-dollar Ponzi scheme.

Kerviel, 33, who is now working at a computer technology company earning euro2,300 ($2,700) a month, plans to argue that he was a scapegoat for the bank, that risky betting practices were commonplace among traders, and that higher-ups knew what he was doing and said nothing as long as he was making money for the bank.

A highlight of the trial is expected to be how Kerviel's legal team will try to demonstrate that claim — which is strongly denied by Societe Generale. Kerviel's lawyer, Olivier Metzner, showed a diagram of the trading room Tuesday depicting how close Kerviel was sitting to his superiors, and Kerviel said he never hid what he was doing.

The former futures trader came to court wearing a crisp dark suit and circles under his eyes. He smiled slightly but stayed silent as a scrum of reporters barraged him with questions outside the courtroom. His attorney responded instead.

"He accepts his responsibility — that's to say having made errors, but errors that are not criminal infractions. They are errors that were authorized and facilitated by the bank," Metzner said.

The former futures trader risks five years in prison as well as a fine of euro375,000 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's aftermath, though it's an amount Kerviel could never pay.

Societe Generale lawyer Jean Veil said the sum was meant as a warning to any trader tempted to go rogue.

"Rogue traders are a tragedy for companies, a tragedy for finance, and as a result that's why Societe Generale's (damages) request should stand as an example," he said.

After a lengthy summing-up of the case, Judge Dominique Pauthe, leading the three-judge panel, woke up the courtroom with a dramatic, "Who are you, Monsieur Kerviel?" Kerviel responded to questions about his schooling and professional career quickly and tersely, almost mumbling into the microphone.

Pauthe pointed out that the psychologist who saw Kerviel described him as a "balanced adult personality." The psychologist also debunked one popular theory, that Kerviel had an inferiority complex because, unlike most of his colleagues, he hadn't attended one of France's creme-de-la-creme universities.

"Did you feel your job was an extreme experience?" the judge asked. "Sometimes," Kerviel responded. Pauthe also referred to a review from one of Kerviel's superiors: "Beware of overheating."

Many questions focused on the bonus for Kerviel that was pending but never paid because the scandal broke: He had asked for euro600,000 and was told it would be euro300,000 instead.

Asked if the sum satisfied him, Kerviel responded, "Not especially." He had never earned more than euro100,000 a year, bonus included.

Kerviel's lawyer, Metzner, asked him if he was a genius, as some people suspected.

"Not at all, I was rather in the lower-middle range of the trading room," he said.

By the hearing's end, the Kerviel mystery was still intact. The trial is expected to last through late June.

In the past, Kerviel has insisted he simply wanted to earn money for the bank, because that was the work ethic his mother, a hairdresser, had instilled in him.

The bank says Kerviel made bets of euro50 billion — more than the bank's market capitalization — on futures contracts on three European equity indices, though his net position appeared unremarkable because he balanced his real trades with fictitious transactions.

Still, an internal report by the bank has said managers failed to follow up on 74 different alarms about Kerviel's activities.

Societe Generale secretly began unwinding his positions on Jan. 21, 2008, when U.S. markets were closed, putting massive pressure on futures markets and exacerbating its losses. The bank revealed its actions three days later, when it also announced subprime-related writedowns and provisions of euro2.05 billion.

Since the scandal, Societe Generale has tightened computer security, reinforced controls and taken more account of the possibility of fraud.

This year the bank has faced another challenge — euro3 billion in exposure to Greek government debt — but reported euro1.06 billion in profit for the first quarter and is forecasting a profitable year.