The Environmental Protection Agency may not be telling the whole truth when it comes to the nation's ethanol mandate, according to the agency's Inspector General.

A report issued Thursday said the agency erred under the law by not providing Congress with regular updates on the impact of its Renewable Fuel Standard, or RFS, withholding key information that policymakers need to make scientific-based decisions about the program's future.

The standard requires refiners to blend an increasing amount of renewable fuels each year into the nation's gasoline and diesel supplies. The standard is being met primarily through the use of corn-based ethanol, while other more advanced renewable fuels are struggling to ramp up to eventually exceed corn ethanol production by 2022.

The program has struggled from a variety of challenges, causing delays in the release of the agency's annual renewable fuel blending requirements and leading the EPA to court. The agency also has struggled to deal with the ethanol "blend wall," the point at which the amount of ethanol being blended into the gasoline supply causes engine damage in vehicles, according to the oil industry. At the same time, more advanced biofuels that don't pose the same hurdles as corn ethanol are years behind their projected targets.

The inspector general's report said the agency, which is required to provide an update on the program's environmental program every three years, has not done so since 2011.

The watchdog said the reporting requirement is necessary to provide an "objective analysis" on the environmental impact and unintended consequences of the program, especially "given conflicting scientific opinions about biofuel impacts, potential impacts outside of the EPA's regulatory control, and divergent RFS interests."

"In addition, the EPA's Office of Air and Radiation has not fulfilled the anti-backsliding requirements for RFS, which are to analyze and address any negative air quality impacts of RFS," it said. Instead, the agency appears to be more concerned with updating its life-cycle analysis to assess the program's benefits in combating climate change, although it is not required to do so, the report said. Climate change is a key priority for President Obama.

As it currently stands, EPA does not have an assessment that meets the requirement to identify whether the RFS "creates any impacts on air quality and, thus, take required measures to mitigate impacts," the inspector general's report said.

"This information is needed to fully inform the EPA, Congress and other stakeholders of the environmental impacts of U.S. biofuel policy," it added.

In June, Congress held a hearing on RFS implementation in which lawmakers expressed "bipartisan interest in receiving more information from the EPA on the environmental impacts, to help assess whether the law's original intent is being achieved and at what cost," the report said.

The inspector general recommends that the EPA issue the report, which it said the agency has agreed to do.