Societe Generale had previously revealed a 4.9bn euros ($7.6bn; £3.8bn) loss, which was said to have been caused by rogue trader Jerome Kerviel who had also worked for the bank.
This second suspect has not been named, but is thought to not be the broker from Societe Generale's Fimat unit who had been questioned.
Rogue trader Jerome Kerviel, who has been named and also held in custody, is under investigation for breach of trust, computer abuse and falsification and he is due in court on Friday in a bid to get bail while the probe into the matter continues.
