Though no longer an adviser to President Donald Trump, billionaire energy investor Carl Icahn may now be the subject of a federal investigation related to his involvement with the Renewable Fuel Standard. The U.S. Attorney's office for the Southern District of New York has subpoenaed Icahn's company, Icahn Enterprises LP, for information on Icahn's work with the president on the RFS, according to a filing with the U.S. Securities Exchange Commission on Nov. 3."The U.S. Attorney's office for the Southern District of New York recently contacted Icahn Enterprises L.P. seeking production of information pertaining to our and Mr. Icahn's activities relating to the Renewable Fuels Standard and Mr. Icahn's role as an adviser to the president," the SEC filing said.