Former IMF chief Dominique Strauss-Kahn was placed under formal investigation on Monday, March 26 by authorities looking into a suspected prostitution ring in the French city of Lille, his lawyer said, following a day of questioning by judges in a closed courtroom, Reuters reported.

The investigation on suspicion of complicity in a pimping operation is the latest judicial headache for the Socialist ex-finance minister. The move could lead to a trial but it falls short of charging him.

Strauss-Kahn, 62, who was a strong contender to be France's next president until he was hit with sex-assault charges, now-dismissed, in New York last May, was allowed to leave the court after being questioned by three judges in the case.

The Lille prosecutor's office said in a statement he was required to post 100,000 euros ($133,300) in bail. He is forbidden to contact witnesses, the press, and others involved in the prostitution case, it said.

Outside the courthouse, Strauss-Kahn's lawyer, Richard Malka, said his client was innocent. The Lille case centers on allegations that a prostitution ring organized by Strauss-Kahn's business associates supplied clients at the city's Carlton Hotel.

Already in the case, eight people, including two Lille businessmen and a police commissioner, have been arrested, and construction firm Eiffage fired an executive suspected of using company funds to hire sex workers.