Federal prosecutors are looking into the role played by Carl C. Icahn, the billionaire investor, in advising the Trump administration on regulatory issues that had the potential to affect the finances of a company he owns.

Mr. Icahn stepped down as an unpaid adviser to President Trump in August, after scrutiny from members of Congress about whether he was influencing regulations on ethanol to benefit his financial investments.

One of Mr. Icahn’s main investment companies, Icahn Enterprises, disclosed in a regulatory filing on Friday that federal prosecutors in Manhattan had served a subpoena seeking information about “Mr. Icahn’s activities relating to the renewable fuels standard and Mr. Icahn’s role as an adviser to the president.” The filing said Icahn Enterprises was cooperating with the prosecutors’ demand for information.

When Mr. Icahn took on the advisory position this year, critics complained that it was a conflict of interest because he owns a big stake in an oil-refinery business called CVR Energy. Indeed, The New York Times reported in March that Mr. Icahn was pressing for a change in a requirement that refiners be held responsible for ensuring that corn-based ethanol is mixed into gasoline.