Sen. Tammy Duckworth on Wednesday asked FBI Director Chris Wray to conduct a criminal investigation of Carl Icahn, a billionaire investor who advised President Trump on regulatory issues.

Duckworth, D-Illinois, alleged that Icahn may have violated a section of criminal law that prohibits executive branch employees from participating in a particular government matter that will affect their own financial interests.

Icahn claims to have never held a formal role in the Trump administration, although he served as a special adviser to the president, a position he resigned from earlier this month after the New Yorker published an article about the conflicts created by his advisory role.

Duckworth wants Wray to probe whether Icahn, 81, violated federal law by pressing the Trump administration to change a requirement that refiners be held responsible for ensuring that corn-based ethanol is mixed into gasoline. One of his investment firms, Icahn Enterprises, owns a large stake in an oil refinery business, CVR Energy.

"Mr. Icahn appears to have abused his role as a special advisor to the President of the United States on issues relating to regulatory reform by participating personally and substantially, through recommendation and the rendering of advice, on a government matter that directly affects his own financial interests," Duckworth wrote in a letter to Wray.

Before Icahn resigned from his advisory role, several Democratic senators had called for him to step down because of potential conflict of interests. But his decision to do so came too late for some critics.

"I am confident that you will understand my grave concern that, if the FBI fails to thoroughly investigate Mr. Icahn's potential violations of criminal conflict of interest statutes, our nation could experience a significant increase in future public corruption, as wealthy individuals are empowered to take advantage of a new 'Icahn loophole' to serve as unpaid officers or employees of the Executive Branch of the United States Government while working to modify Federal programs and policies in a manner that directly benefits their own personal financial interests," Duckworth wrote to Wray, who officially became Trump's FBI director August 2.