France’s stock market regulator opened an investigation into speculation that affected trading in shares of Societe Generale SA, said the agency’s president, Jean-Pierre Jouyet.

Societe Generale, France’s second-largest bank, on Aug. 10 denied “all market rumors” and asked France’s Autorite des Marches Financiers to open a probe. Speculation that France’s creditworthiness was in doubt sent the shares tumbling 15 percent that day.

“We’re investigating the unfounded rumors that hit specific stocks this week,” Jouyet said in an interview on RTL Radio today. “I can tell you that the secretary general of the AMF, Thierry Francq, has opened an investigation into the rumors that affected Societe Generale.”

France, Spain, Italy and Belgium banned short-selling of financial stocks beginning today. The ban is aimed at stabilizing the market, reducing the volatility in some stocks and preventing manipulation based on false rumors, Jouyet said in the interview.