(Reuters) - Six Democratic U.S. senators have asked billionaire investor Carl Icahn and Environmental Protection Agency Administrator Scott Pruitt to explain how an Icahn-owned refinery secured a valuable EPA exemption from the nation’s biofuels law.

FILE PHOTO: Billionaire activist-investor Carl Icahn gives an interview on FOX Business Network's Neil Cavuto show in New York, U.S. on February 11, 2014. REUTERS/Brendan McDermid/File Photo

The request, made in letters sent late on Tuesday and reviewed by Reuters, adds pressure on the embattled EPA chief over his pro-business policies, as well as on Icahn, whose dual role last year as an investor and presidential adviser is being investigated by the Justice Department.

Reuters reported last week that EPA granted a small refinery hardship waiver from the nation’s biofuel laws to an Oklahoma refinery operated by Icahn’s CVR Energy Inc, allowing it to avoid tens of millions of dollars worth of costs related to the U.S. Renewable Fuel Standard.

“We ... are troubled that a company that is owned by a billionaire former ‘special adviser’ to the President who is currently under investigation by federal prosecutors ... has now received an ‘economic hardship waiver’,” the senators wrote in the letters to Icahn and Pruitt.

Icahn did not respond to a request for comment. His attorney, Jesse Lynn, declined to comment.

The senators asked Icahn and Pruitt for details on the waiver, including records of discussions between both men in the months before it was granted.

The letters were signed by Democrats Elizabeth Warren of Massachusetts, Sheldon Whitehouse of Rhode Island, Sherrod Brown of Ohio, Tammy Duckworth of Illinois, Tammy Baldwin of Wisconsin, and Amy Klobuchar and Tina Smith of Minnesota.

The RFS requires refiners to add biofuels such as corn-based ethanol into their gasoline and diesel, or to buy blending credits from rival companies that do - a policy intended to help farmers, cut petroleum imports, and reduce air pollution.

EPA has the authority to exempt small refineries of less than 75,000 barrels per day of capacity if they can prove they are struggling financially with these regulations.

The agency has said it has granted more than two dozen such waivers in recent months, but has refused to confirm the recipients, saying the information is business sensitive. The EPA has in the past tended to grant less than 10 per year, according to a former official.

Reuters has reported that Andeavor, one of America’s biggest refining companies, was among the other companies that have received hardship waivers from Trump’s EPA for its small refineries.

An early supporter of Trump’s 2016 presidential run and a key supporter on Wall Street, Icahn had met with Pruitt when Pruitt was being vetted in late 2016 for the EPA administrator job, and later served as a special regulatory adviser to the Republican president.

Icahn stepped down from his advisory role last August after lawmakers cited potential ethical problems.

Currently, Icahn is under investigation by the U.S. Department of Justice for his role in influencing biofuels policy while serving as Trump’s adviser. Some U.S. lawmakers have expressed concern that Icahn may have used his presidential access to benefit his investments, a charge Icahn has rejected.