Elizabeth Warren and 7 other Democratic senators are calling on U.S. regulators to open an investigation into Carl Icahn’s activities in the market for renewable fuel credits, citing “troubling” evidence concerning the billionaire investor and adviser to President Donald Trump.

Warren and the other senators wrote to the heads of the Commodity Futures Trading Commission, Securities and Exchange Commission and Environmental Protection Agency, asking them to probe “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.” See the text of the letter.

The senators cite Icahn’s February proposal to the Trump administration to modify the renewable fuel credit program, and note that prices for those credits fell by about a third following Icahn’s proposal. Icahn and CVR Energy Inc. CVI, +0.65% — an oil refining company in which Icahn owns a majority stake — reaped a $50 million windfall, the senators wrote.

Warren and other senators including Sherrod Brown of Ohio and Thomas Carper of Delaware wrote they 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.”

“But,” they continued, “the publicly available evidence is troubling,” and asked the regulators to look at the extent and precise nature of Icahn’s communications with Trump and other administration officials during and after the presidential transition, among other issues. The agencies are not obligated to act on the senators’ request.

Icahn has said his advocacy on renewable fuels is not self-serving, since it would benefit a broad swath of the refining industry, including CVR’s competitors, as Reuters has written.

A White House spokeswoman said that Icahn does not have a policymaking role in the Trump administration.

“As we made clear in our initial press release concerning Mr. Icahn’s intended future role, Mr. Icahn will be speaking with the president solely in his individual capacity and will not be serving as a federal employee nor as a special government employee and will not have any specific duties,” the spokeswoman said. “He is simply a private citizen whose opinion the president respects and whom the president speaks with from time to time.”

The letter was signed by Sens. Warren, Carper, Brown, Debbie Stabenow of Michigan, Sheldon Whitehouse of Rhode Island, Tammy Baldwin of Wisconsin, Tammy Duckworth of Illinois and Amy Klobuchar of Minnesota.