The U.S. Environmental Protection Agency’s scientific integrity watchdog is reviewing whether EPA chief Scott Pruitt violated the agency’s policies when he said in a television interview he does not believe carbon dioxide is driving global climate change, according to an email seen by Reuters on Friday.

Lawyers for environmental group the Sierra Club had asked the EPA’s Office of Inspector General to check whether Pruitt violated policy when he told a CNBC interviewer on March 9, “I would not agree that it’s a primary contributor to the global warming that we see.”

The EPA Inspector General’s office responded to the Sierra Club on Thursday in an email, saying it had referred the matter to the EPA’s Scientific Integrity Officer, Francesca Grifo, for review.

“If after the SIO review, she concludes there is some aspect of the letter itself, or her findings or conclusions that she believes are appropriate for further consideration by the OIG, she will so notify the OIG,” the email stated.

A spokesman for the EPA did not immediately respond to a request for comment, and efforts to reach Grifo were not immediately successful.

The EPA website says its scientific integrity policy requires EPA officials and staff to ensure the agency’s work respects the findings of the broader scientific community.

An overwhelming majority of scientists think that carbon dioxide emissions from the burning of fossil fuels are a major contributor to global climate change, triggering sea level rise, droughts and more frequent violent storms.

The request by Sierra Club ramps up tension between the U.S. environmental movement and the administration of President Donald Trump, who has called global warming a hoax meant to weaken the U.S. economy and has packed his cabinet with people who question the science of climate change.

Grifo is a biologist who was hired by former President Barack Obama’s administration. Before she joined the EPA in late 2013, she oversaw scientific integrity at the Union of Concerned Scientists, a non-partisan group advocating stronger environmental protections.

Pruitt, a former attorney general for Oklahoma, sued the EPA more than a dozen times over its regulations as top prosecutor for the oil and gas producing state.