Jerome Kerviel graduated from the middle-ranking Universite Lumiere in Lyon with a masters degree in finance, he joined Soc-Gen in 2000, landing a job in the bank’s compliance department, where he built up a vast knowledge of its control procedures which were designed to stop rogue traders. He was promoted to the bank’s Delta One trading team in 2005, which specializes in the futures markets, but made little impact and was restricted to so-called “plain vanilla” trading — financial jargon for the most basic type of trading.

In December he allegedly decided to start trading by himself, using up to 60 billion pounds of the bank’s money to bet on whether European stock markets would go up or down. He managed to break even, it is claimed, and in January, emboldened by his earlier, undetected actions, he started betting again, but this time the markets took a nosedive and his bets went spectacularly wrong.

Mr. Kerviel was allegedly able to hack into the bank’s computers to hide his trading, making him “invisible” until a basic slip-up on Friday, when he failed to disable the bank’s automatic alert system and his irregular trading suddenly showed up.

At the time, the losses were around 1.2 billion pounds, but as the markets plunged in the early part of this week, the size of the loss trebled before the bank was able to shut down the fake accounts Mr. Kerviel had allegedly set up.