The U.S. District Court for the Northern District of Illinois in Chicago froze the U.S. assets of Mark Karpeles and two associated companies. The move allows lawyers to begin demanding documents from the three parties to investigate what they allege is a huge fraud.

Tuesday’s ruling covers the CEO and Mt. Gox’s U.S. subsidiary and Japanese parent company, Tibanne KK, but it does not cover the failed Mt. Gox itself.

Chris Dore, a partner at Edelson PC in Chicago, is representing Gregory Greene. Greene filed the Chicago lawsuit against Mt. Gox on Feb. 27. In it, he alleges that he lost around $25,000 worth of bitcoins in the collapse.

Dore alleges that he suspects there may be ongoing fraud. “Money is moving around as we speak, bitcoins are moving around accounts associated with Mt. Gox. Something is not right so we urgently need to get to the bottom of this quickly.”

The suit may turn into a class action suit: “I’ve been contacted by at least 300 people from around the world who could be class members, at least the U.S. citizens among them,” Dore said.