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Prosecutors are investigating whether billionaire businessman Carl Icahn pushed for a federal policy change that would have benefited one of his investments while he was serving as an adviser to President Donald Trump.

The U.S. attorney's office in Manhattan has issued subpoenas to Icahn's company and another company, CVR Energy, in which he has an 82 percent stake, both firms disclosed in regulatory filings. CVR is a publicly traded company that specializes in refining and is valued at nearly $3 billion.

The companies said they are cooperating with the subpoenas, which were first reported by Bloomberg. "The U.S. Attorney's office has not made any claims or allegations against us or Mr. Icahn," Icahn Enterprises L.P. said in its filing.

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Donald Trump greets Carl Icahn, billionaire activist investor, at a campaign event in April 2016. Victor J. Blue / Bloomberg via Getty Images file

Icahn, who backed Trump during the campaign, was named a special adviser on regulation last December. He quit in August to avoid what he called "partisan bickering" about his position in the administration.

His resignation followed an investigation by CNBC into potential conflicts of interest and came just before a magazine report questioning whether he had broken any laws.

The matter centered on the Renewable Fuel Standard, a program that requires refiners to blend renewable biofuels into gasoline and diesel. Icahn wanted changes that would have helped CVR Energy, CNBC reported.

The U.S. Attorney's office declined to comment on the scope of the probe, but Icahn LP said in its filing that investigators want "information pertaining to our and Mr. Icahn's activities relating to the Renewable Fuels Standard and Mr. Icahn's role as an advisor to the President."

Icahn did not immediately return a call for comment but when he resigned, he denied any conflict.

"I never had a formal position with your administration nor a policymaking role. And contrary to the insinuations of a handful of your Democratic critics, I never had access to nonpublic information or profited from my position, nor do I believe that my role presented conflicts of interest," he wrote to Trump.

"Indeed, out of an abundance of caution, the only issues I ever discussed with you were broad matters of policy affecting the refining industry. I never sought any special benefit for any company with which I have been involved, and have only expressed views that I believed would benefit the refining industry as a whole," he added.