The Environmental Protection Agency needs to explain why it didn't compile years of reports detailing the challenges facing its renewable fuel program and ethanol mandate, said Rep. James Sensenbrenner, a vocal critic of the program.

The Wisconsin Republican, who serves on the House Science, Space and Technology Committee's oversight panel, sent a letter to EPA Administrator Gina McCarthy Thursday, asking her to detail the reasons why the agency did not fulfill the requirement to conduct the studies and deliver them to Congress.

The letter follows last month's EPA inspector general report that showed the agency has not issued the required reports on the ethanol mandate since 2011.

"The EPA has a statutory requirement to produce these reports," Sensenbrenner said. "It is vital that members of Congress have access to the agency's findings while evaluating the strengths and weaknesses of our national biofuels mandate in order to take sound actions on behalf of the American consumer."

The EPA biofuel program, known as the Renewable Fuel Standard, requires refiners to blend ethanol and other renewable fuels into the nation's gasoline and diesel supplies. The agency is required to produce reports detailing the progress made by the fuel program as well as any challenges facing it.

It also is required to produce a report on whether biofuel use is causing environmental harm, rather than improving greenhouse gas emissions, which is one of the program's goals.

"As noted by the Office of Inspector General, EPA hasn't completed a triennial congressional impact report since 2011 and never issued a backsliding study to determine if the RFS and our national biofuels mandate adversely affects air quality," Sensenbrenner wrote in the letter.

"Additional and updated research and analysis allows lawmakers to better gauge the strengths and weaknesses of policy we enact, and science-based decision making is vital when evaluating our biofuel mandate," the letter continued.

He wants McCarthy to provide answers to a series of questions in a month.

One area where he wants answers is on EPA's ability to meet deadlines. Sensenbrenner wants to know "how confident" the agency is that it can produce its next triennial report in 2018. He wants to know "what specifically is EPA doing to ensure this deadline is met?"

He also wants to know if McCarthy supports the EPA air office's rationale for not doing a report on the lifecycle greenhouse gas emissions of the renewable fuel program.

"Although EPA says some of the required reports were not produced due to scarce resources and other priorities, the Office of Air and Radiation stated that in regards to lifecycle greenhouse gas (GHG) emissions, the state of the science (since 2010) has not changed enough to necessitate an updated study on the impacts of the renewable fuels program," he wrote.