Billionaire Carl Icahn's investment firm is cooperating with an inquiry by federal prosecutors into the activist's work for President Donald Trump, an unpaid advisory post he relinquished after questions over whether he improperly benefited from energy policies he recommended to the White House.

The U.S. Attorney's Office in Manhattan "has not made any claims or allegations against us or Mr. Icahn," Icahn Enterprises LP  (IEP) - Get Report said in a regulatory filing this month. "We maintain a strong compliance program and, while no assurances can be made, we do not believe this inquiry will have a material impact on our business."

A representative for Icahn didn't immediately respond to a message seeking comment.

The prosecutor's subpoena was disclosed about six months after Democratic senators including Elizabeth Warren of Massachusetts regulators to investigate whether Icahn's "actions in the market for renewable fuel credits" had violated any laws. 

As the majority owner of CVR Energy Inc. (CVI) - Get Report , Icahn had bet in 2016 that the price of renewable fuel credits would decline, the senators said in the May 5 letter. Afterward, while serving as Trump's unpaid adviser, he recommended policies and personnel appointments -- including that of Environmental Protection Agency Director Scott Pruitt, a critic of the credits program -- that led to a drop, netting $50 million, they wrote.

"We have no way of knowing at this time whether Mr. Icahn made any of his renewable fuel credit trades or decisions about trades based on material, non-public information or otherwise manipulated the market," the senators said. "But the publicly available evidence is troubling."

Three months later, Icahn sent a resignation letter to Trump, saying he didn't want "partisan bickering" about his role to cloud the administration. "I sincerely regret that because of your extremely busy schedule, as well as my own, I have not had the opportunity to spend nearly as much time as I'd hoped on regulatory issues," he added.

Less than two months later, however, Warren and another Democrat were raising more questions about Icahn's work with the White House, asking Treasury Secretary Steve Mnuchin whether the investor had influenced the Financial Stability Oversight Council's decision to lift American International Group Inc.'s (AIG) - Get Report designation as a systemically important financial institution.

The label, applied after the insurer received a $182 billion bailout during the height of the 2008 financial crisis, subjected AIG to stricter regulatory oversight and required executives to maintain higher capital reserves.

Icahn, who holds a stake in the company, had urged former CEO Peter Hancock to break it up in order to escape that designation, though he stopped doing so publicly when Hancock stepped down and was replaced by Michael Duperreault, a lieutenant of legendary AIG chief Maurice "Hank" Greenberg.

In letters written to each of the council's members individually, Warren and Sen. Sheldon Whitehouse of Rhode Island, asked whether Icahn had a role in the decision, which was made in a hastily convened meeting and with scant explanation.

Queries in a previous letter to Mnuchin about Icahn's communications with council members weren't answered, she noted, "meaning the existence of or precise extent and nature of any contacts is still unclear."

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