The former IMF chief, Dominique Strauss-Kahn, was behind bars in Lille after being arrested "by appointment" to answer questions about an international prostitution ring.
Strauss-Kahn, 62, faced up to four days of interrogation over his alleged participation in orgies with prostitutes in Lille, Brussels, Paris and Washington between 2009 and 2011.
An investigation into the "Carlton affair" has already led to formal accusations against a senior French police officer, two hotel managers, several businessmen and a club and a brothel owner known as "Dodo la Saumure" (or Dodo the pickled mackerel, saumure being French slang for pimp).
Strauss-Kahn, a former Socialist finance minister who was once favourite to win this year's presidential election, was first publicly linked with the investigation last northern autumn. He returned to France last August after American prosecutors dropped criminal charges that he had sexually assaulted a chambermaid in a Manhattan hotel in May.
Strauss-Kahn could face formal accusations of complicity in organised pimping and receiving embezzled company funds. Two businessmen, Fabrice Paskowski and David Roquet, who are known as DSK's "friends in the north", are alleged to have paid for the orgies from their companies' funds without authorisation.
DSK admits that he attended the orgies, but denies any part in their organisation. Paying for sex is not illegal in France; organising paid-for sex is.
The former IMF chief says that he had no idea that the parties were funded by two large firms or that the women present were call-girls. His lawyer, Henri Leclerc, said in December: "At these kinds of parties you're not always dressed, and I challenge you to distinguish a naked prostitute from any other naked woman."
One of the parties, featuring young women flown in from northern France, was held in a luxury hotel in the United States capital on May 12-14 last year, the day before Strauss-Kahn was accused of assaulting the chambermaid Nafissatou Diallo in the Manhattan Sofitel.
DSK has been asking for months to meet investigators to give his side of the "Carlton affair" (named after the top hotel in Lille alleged to have been the epicentre of the call-girl ring). To avoid arrest in front of TV cameras, he travelled to Lille yesterday.
His prosecution is far from certain. Investigators may decide that, even if he knew that the women were call-girls, there is no proof that he organised the events. It could also be difficult to prove that he knew that the parties were financed with embezzled funds.
Witness statements by the call-girls have been leaked in the French press.
Mounia R., 38, said that she flew to Washington to take part in parties with DSK for €2400 ($3800) a weekend. She said that she had also consented to a "violent" sexual encounter with DSK on another occasion for €900. "Everyone present must have been well aware that I was being paid," she told investigators.
Another woman, Florence V., said that she was "almost sure" that DSK assumed that she was unpaid.
An article in Le Monde suggested the parties may have been organised to allow DSK to satisfy his sexual appetites without falling foul of the Washington establishment after his affair with a Hungarian IMF official in 2008.