French prosecutors say they have opened a preliminary investigation into rape allegations against former International Monetary Fund chief Dominique Strauss-Kahn.

Prosecutors in Lille said Monday that Strauss-Kahn may have been involved in a gang rape during a sex party in December 2010 in a Washington, DC hotel. Authorities say three other people were allegedly involved in the incident.

A prostitute testified that Strauss-Kahn and friends forced her to have sex in a group. She has not filed a formal complaint.

The former IMF chief has denied the allegations and said he is not guilty of any violence. He also is facing charges in Lille in connection with a prostitution case.

A New York hotel maid accused Strauss-Kahn of sexually assaulting her last year in the luxury hotel where she worked. Those charges were dropped when prosecutors questioned the woman's credibility, but not before Strauss-Kahn was forced to step down as IMF chief.

Earlier this month, he countersued the maid, Guinea native Nafissatou Diallo, saying she cost him not only his IMF job but also political opportunities, including the French presidency.

Diallo's lawyers called the $1-million lawsuit a publicity stunt.

French Socialists had earlier touted Strauss-Kahn as a likely presidential candidate. Instead the party backed newly elected President Francois Hollande, who defeated former leader Nicolas Sarkozy in a runoff vote earlier this month.