Environmental Protection Agency Administrator Scott Pruitt assured Democratic senators that he did not make promises to billionaire investor Carl Icahn about changing a policy that requires oil refineries to blend ethanol into gasoline.

Pruitt, the Associated Press reported Thursday, responded to letters from five senators looking into potential conflicts of interest involving Icahn, who resigned in August as a special adviser to President Trump on regulatory reform.

Last month, Icahn, 81, resigned from his advisory role after the New Yorker published an article about the conflicts created by his informal government job.

At least one Democrat, Sen. Tammy Duckworth of Illinois, has asked FBI Director Chris Wray to probe whether Icahn 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.

Icahn unsuccessfully tried to pressure the government to change the rules regarding the "point of obligation," trying to forgive CVR from the responsibility of blending the ethanol, according to the New Yorker.

In a letter to Sen. Sheldon Whitehouse, D-R.I., dated Monday, Pruitt wrote that he met with Icahn during Pruitt's confirmation process, but offered no assurances.

"I made no assurances with regard to the point of obligation or any other substantive issue," Pruitt wrote.