Eight Democratic senators called Tuesday for regulators to probe potential insider trading by billionaire investor Carl Icahn in the market for renewable fuel credits. In a letter to the heads of the Commodities Futures Trading Commission, Securities and Exchange Commission and the Environmental Protection Agency, senators including Massachusetts Democrat Elizabeth Warren said they had no way of knowing if Icahn made certain trades based on non-public information, "but the publicly available evidence is troubling." In addition to Warren, the letter was signed by Sens. Debbie Stabenow of Michigan, Tom Carper of Delaware, Sherrod Brown of Ohio, Sheldon Whitehouse of Rhode Island, Tammy Baldwin of Wisconsin, Tammy Duckworth of Illinois and Amy Klobuchar of Minnesota.