On Monday, the senators sent a letter to Icahn, the Office of Government Ethics and the Department of Justice in which they expressed their concerns about Icahn “breaking federal conflict of interest laws.”

Icahn, 81, is serving as special adviser on regulatory matters and has been pushing to get rid of a federal requirement that refiners must ensure that ethanol is mixed with gasoline, instead pushing for gas station owners to be held responsible.

The Jewish billionaire is a majority investor in CVR Energy, a Sugar Land, Texas-based oil refiner that handles gasoline but does not distribute it. The reform Icahn is advancing would have saved CVR Energy over $200 million in 2016, according to The New York Times.

“Reports suggest you are unfairly using your position in the White House and your access to President Trump to influence that nation’s regulatory policies in order to serve your own interests and not those of the American people,” the letter reads.

The signers are Sens. Sheldon Whitehouse of Rhode Island; Elizabeth Warren of Massachusetts; Patrick Leahy of Vermont; Sherrod Brown of Ohio; Jeffrey Merkley of Oregon; Tammy Baldwin of Wisconsin, and Tammy Duckworth of Illinois. The letter included 15 questions about Icahn’s role in the Trump administration.

Icahn told The New York Times that he was only serving as an adviser.

“I’m not making any policy,” Icahn said. “I am only giving my opinion.”

In December, a White House spokeswoman emphasized that Icahn was advising the president as a private citizen.

“He is simply a private citizen whose opinion the president respects and whom the president speaks with from time to time,” Kelly Love said in a statement. “Mr. Icahn does not have a position with the administration nor a policy-making role.”

Icahn, an early supporter of Trump’s candidacy, is the founder of Icahn Enterprises, a diversified conglomerate based in New York City formerly known as American Real Estate Partners. He has also held substantial or controlling positions in numerous American companies over the years, including RJR Nabisco, Texaco, Philips Petroleum, Western Union, Gulf & Western, Viacom, Revlon, Time Warner, Motorola, Chesapeake Energy, Dell, Netflix, Apple and eBay.

Icahn is a major giver to Mount Sinai hospital in New York City, among other philanthropic endeavors. In 2012, he donated $200 million to the renamed Icahn School of Medicine there.

In addition, Icahn established seven Icahn Charter Schools in the Bronx borough of New York.