The assistant of rogue trader Jerome Kerviel is being investigated for complicity in his former boss' trading scandal that cost one of France's largest banks billions of euros.

Thomas Mougard was charged Friday as an "assisted witness," in the case — a legal status for witnesses who may be implicated themselves.

The French bank Société Générale said it lost a staggering €4.9bn (£3.9bn, $7.6bn) when Kerviel manipulated the bank's trading systems to cover risky and unauthorized positions in the derivatives market.

Investigators have already filed preliminary charges against Kerviel for forgery, breach of trust and illegally accessing the bank's computers.

An internal investigation by SocGen released to the public in May said it found "signs of internal complicity" by Mougard.

Kerviel admits to gaming the system, but claims his managers knew of his dealings and never stopped him so long as he was making money for the bank. His lawyers assert that SocGen was in fact responsible for the colossal losses after higher-ups "took the decision to unwind Mr. Kerviel's positions as shares were tumbling on the world's markets in January."

Kerviel is presently free on bail pending trial. In April, he scored himself a new IT job as a computer consultant after the judge in his case changed the condition of his release so he could take the post. ®

Related stories

• Second French rogue trader case sparks risky thinking (3 November 2008)
• France's €4.9bn rogue trader gets IT job (25 April 2008)
• ID cards target the young, MySpace targets MySpace (1 February 2008)
• Low-tech hack was behind $7.2bn SocGen fraud (30 January 2008)
• Rogue trader blows sox off control systems (25 January 2008)