“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 the Securities and Exchange Commission, the Commodity Futures Trading Commission, and the Environmental Protection Agency.

The letter, a copy of which was seen by Reuters, was signed by Democratic Senators Elizabeth Warren of Massachusetts, Sheldon Whitehouse of Rhode Island, Sherrod Brown of Ohio and five others. The letter acts as a request, and the agencies are not required to act upon it.

Icahn became an unpaid adviser to Trump on regulation shortly after November’s presidential election. In February Icahn proposed a change to the U.S. biofuels program that would ease the regulatory burden on oil refining companies – including CVR Energy Inc, in which he owns a majority stake.

CVR Energy stock fell 1.6 percent to $21.93 after news about the senators’ letter.

CVR had accumulated a large short position in biofuels blending credits, called Renewable Identification Numbers or RINs which many refiners are required to purchase under the biofuels law. The company would have been in a strong position to profit if RIN prices fell, according to a Reuters review of securities filings last month.

RIN prices dropped sharply after Icahn’s proposal on the biofuels regulation was reported, the senators pointed out in their letter.

“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 wrote.

“But the publicly available evidence is troubling, and based on this evidence, we ask that your agencies investigate whether Mr. Icahn’s conduct violated any laws under your jurisdiction,” they said.

CVR Energy’s refining unit posted a net gain of $6.4 million associated with the credits in the first quarter of this year, a $50 million turnaround from the year-ago period when CVR shelled out $43.1 million, the company said in a filing.

Icahn has previously dismissed accusations that his proposal to the White House to change the biofuels program was self-serving, saying the overhaul would help other refining companies too, including his competitors.

The U.S. Renewable Fuels Standard requires increasing volumes of biofuels to be blended into the nation’s gasoline and diesel every year. It places that responsibility mainly on refiners, who are required to either blend the fuel themselves or purchase credits from others who do it for them.

Icahn’s proposal in February had called for regulators to shift the responsibility for blending from refiners – many of which lack sufficient blending capacity – to terminal operators. The White House is considering the proposal.