Democratic Senator Tammy Duckworth on Wednesday asked the U.S. Environmental Protection Agency's Office of Inspector General to investigate why the agency vastly expanded its use of waivers to exempt small refineries from the nation's biofuel law.

The request, made in a letter from Duckworth's office to Acting Inspector Charles Sheehan, follows a May 16 Reuters report that the EPA decided to expand the waiver program months before a 2017 court decision it has often cited to justify the move to the corn lobby.

Oil refiners say the Small Refinery Exemption (SRE) program is crucial to help limit regulatory compliance costs, but the corn industry has said it suspects President Donald Trump's EPA is using the program as a political tool to help allies in the energy industry at the expense of farmers.

"Recent document disclosures reveal that the EPA misled Members of Congress, industry and the public in regard to the agency's motivations and justifications for its SRE policy," the letter stated. "This deception by EPA political appointees may indicate improper motives and conflicts of interest and it warrants a thorough review by the EPA OIG."

The U.S. Renewable Fuel Standard requires refineries to blend billions of gallons of biofuels like corn-based ethanol into their fuel each year, but small facilities that can prove compliance would cause financial hardship can seek waivers.

The Trump administration's EPA granted 35 such waivers for 2017, up sharply from seven in the final year of the Obama administration - saving the refining industry hundreds of millions of dollars but infuriating the corn lobby, which argues the waivers have threatened demand for ethanol.

Some of the waivers granted by Trump's EPA have gone to small refineries owned by oil giants like Exxon Mobil and Chevron Corp. as well a facility owned by billionaire investor Carl Icahn, who had briefly advised Trump on environmental regulation.