A group of eight Democratic lawmakers including Sen. Elizabeth Warren on Tuesday demanded a probe into billionaire corporate raider-turned-activist investor and President Trump adviser Carl Icahn's activities in the U.S. biofuel market, focusing on possible insider trading.

"We are writing to request that your agencies investigate whether Carl Icahn violated insider trading laws, anti-market manipulation laws, or any other relevant laws based on his recent actions in the market for renewable fuel credits," the senators said in a letter to the heads of three regulators on Tuesday.

Specifically, the Massachusetts Democrat and the other lawmakers point out that Icahn, as an unpaid adviser to Trump, recommended personnel and policies that they assert caused the price of renewable fuel credits to drop.

The investments resulted in a $50 million turnaround from Icahn's initial investment, the senators alleged in their letter. Icahn's office did not return a request for comment.

At the center of their concerns is Icahn's majority control of CVR Energy (CVI) , an oil refiner that has a significant interest in the renewable fuel credit market. CVR Energy's shares were down slightly Tuesday afternoon on the news of the letter, trading down 0.04% at $22.28 a share.

The letter was made out to Commodity Futures Trading Commission chief Christopher Giancarlo, Securities and Exchange Commission Chairman Jay Clayton and Environmental Protection Agency head Scott Pruitt, all Trump Administration appointees.

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