Société Générale employee Jérôme Kerviel met with worldwide notoriety as the so-called 'rogue trader' who lost the bank almost 5 billion euros in reckless trading bets in 2007. He was sentenced to five years in prison – two of them suspended - and a staggering fine of 4.9 billion euros, a sentence upheld after he lost an appeal in October 2012. The bank has consistently claimed that Kerviel acted alone and kept his high-risk bets secret from his superiors. But in this interview with Mediapart, a key witness to Kerviel's appeal case, but who was never called to testify, explains why Kerviel's activities were necessarily known to the bank, which at best turned a blind eye. What's more, he tells Martine Orange, the concrete proof of this is still available in logged and stored data - but not for long.