DRW filed the lawsuit in the U.S. District Court for Northern Illinois, seeking a preliminary injunction to stop what it says would be an "unconstitutional enforcement action" by the CFTC alleging "manipulative activities from August 2010 through September 2011" that would cause DRW irreparable harm. A spokesman for the agency declined to comment. The firm says in the lawsuit documents that it has been under investigation by the agency for about two years and first became aware of the probe in August 2011 when the CFTC asked it for documents related to its trading of an interest rate swap futures contract on the Nasdaq OMX Futures Exchange.