A group of Democratic senators on Tuesday urged federal regulators to investigate whether Donald Trump adviser Carl Icahn had engaged in insider trading and other violations.

A letter from the senators to regulators focused on Icahn’s petroleum refining company, CVR Energy, and its treatment of renewable fuel credits, called Renewable Identification Numbers or RINs. It raises the possibility that Icahn may have taken advantage of his connections to the White House in order to save his company hundreds of millions of dollars in credits.

In December 2016, Trump named Icahn a “special adviser to the president” on regulatory matters, but noted that Icahn was serving “in his individual capacity and will not be serving as a federal employee or a Special Government Employee and will not have any specific duties.”

The letter was signed by Sens. Elizabeth Warren (D-MA), Debbie Stabenow (D-MI), Thomas Carper (D-DE), Sherrod Brown (D-OH), Sheldon Whitehouse (D-RI), Tammy Baldwin (D-WI), Tammy Duckworth (D-IL) and Amy Klobuchar (D-MN).

The RIN market is designed to benefit companies that add biofuels like ethanol to their fuel products, and require companies like Icahn’s, which doesn’t, to pay for credits.

“Over the course of 2016, including the months immediately prior to and following President Trump’s election, CVR Energy delayed purchases of necessary renewable fuel credits and instead sold millions of them,” the senators wrote.

The letter cited an April 12 Reuters article, which describes CVR’s actions as “a bet that it could buy the credits it would need later at lower prices.”

Reuters described several events that drove down the cost of RINs, in turn potentially benefitting Icahn: the nomination of Scott Pruitt as EPA administrator — about whom Icahn said Trump had consulted him — Icahn’s own naming as an adviser to Trump on regulation in December 2016 and news in February that Trump would consider a proposal by Icahn on biofuels regulation.

Reuters reported in February, citing unnamed people familiar with the plan, that the President “intends to revamp the national biofuel program to ease regulations on oil refiners.”

The senators noted that, after the February news broke, RIN prices dropped to a 17-month low, a 70 percent decline from November 2016.

In February, a similar group of senators, minus Carper and Klobuchar and plus Al Franken (D-MN), raised concerns about Icahn in a letter to the EPA and White House Counsel Don McGahn.

“With a sprawling business empire and potentially unlimited portfolio in the Administration to address ‘strangling regulations,’ Mr. Icahn’s role presents an unacceptable risk of further real or potential conflicts of interest absent immediate and thorough steps to address them,” they wrote.

Tuesday’s letter acknowledges that Icahn’s conflicts of interest could extend to any potential investigation of Icahn. It is addressed to Pruitt, as well as the acting chair of the Commodities Futures Trading Commission and Jay Clayton, chair of the Securities and Exchange Commission.

“Mr. Icahn was reported to be heavily involved in interviewing candidates to for [sic] SEC Chairman,” they wrote, “and SEC Chairman Clayton has acknowledged that he met with Mr. Icahn after he was nominated as SEC Chair.”

Read the letter below: