A government watchdog group, Public Citizen, said on Wednesday it will ask lawmakers to investigate whether billionaire investor Carl Icahn should have been subject to lobbying disclosure laws when he advised President Donald Trump to overhaul the US biofuels program.

Icahn, an unpaid adviser to Trump on regulation, submitted a proposal to Trump last month to change the US Renewable Fuel Standard by shifting the burden of blending biofuels into gasoline away from oil refining companies, and further down the supply chain to marketers.

Public Citizen said that, because Icahn owns a controlling stake in a refinery that could benefit from the proposed change, he may have been required by a 1995 lobbying disclosure law to disclose his discussions with Trump on the subject as lobbying. The group said it would make its request for a probe in a letter to Congress later on Wednesday.

“All of this has occurred with no record of any (Lobbying Disclosure Act) filings by or on behalf of Mr. Icahn,” Public Citizen said in a copy of the letter provided to Reuters.

The Renewable Fuel Standard, signed into law by former President George W. Bush, requires refiners to mix increasing levels of renewable fuels into gasoline and diesel each year — a requirement that many refiners say costs them millions of dollars.

Icahn owns an 82 percent stake in refiner CVR Energy Inc., which, along with other refining companies, has urged the Environmental Protection Agency to shift the blending obligation away from them.

Icahn has said the change would benefit not just his company, but the entire refining industry.

Icahn has disclosed his role as a Trump adviser to the Securities and Exchange Commission, but he has not registered as a lobbyist. Several Democratic lawmakers have said they want more information about his role in the Trump administration.

Last week, the head of a US biofuels group said Icahn told him that Trump was readying an executive order to change the point of obligation for blending under the biofuels program, something both the White House and Icahn have denied. The White House has said it is reviewing Icahn’s proposal and has not yet taken a position.