A French court ordered the detention of Jerome Kerviel, the rogue trader blamed by Societe Generale for huge losses, as police questioned a second trader to establish whether he knew about Mr Kerviel's illicit deals.
The Paris appeals court ruled today that Mr Kerviel should be held in custody for the duration of the investigation because of a risk he might try to abscond or act in concert with any accomplices he may have had.
Mr Kerviel was taken to La Sante prison, close to the centre of Paris, where high-profile suspects such as businesss leaders and politicians are often held while under investigation.
His lawyer, Elisabeth Meyer, was seen by reporters at the scene leaving the hearing in tears. She said she would appeal against the decision.
A second trader was also being questioned by police, a source familiar with the matter told Reuters. This trader worked at Newedge, a brokerage that executed orders on behalf of Mr Kerviel, the source said.
If investigators can show that others knew about Mr Kerviel's trading activities it would be a dramatic new development in the case.
SocGen has maintained that Mr Kerviel acted on his own. But the lone trader has told police that it was impossible that his supervisors could have been entirely in the dark about his activities.
A separate legal source, who confirmed earlier that police were questioning an unidentified trader, said his initial 24-hour detention period had been extended by another day.
The brokerage is a SocGen subsidiary formerly known as Fimat but renamed Newedge this year after it was wholly acquired by Calyon Financial. Its offices were raided by police on Thursday.
French police are investigating a trading loss of €4.9 billion ($7.1 billion) at Societe Generale which was revealed last month. SocGen has blamed Mr Kerviel for the losses.