U.S. environmental group the Sierra Club has asked the Environmental Protection Agency¡¯s inspector general to investigate whether the agency¡¯s head, Scott Pruitt, violated internal policies when he said he did not believe carbon dioxide was a major contributor to climate change, according to a letter seen by Reuters on Wednesday.

Lawyers for the Sierra Club wrote to the EPA¡¯s Office of Inspector General on Tuesday asking the independent watchdog to check whether Pruitt violated the EPA¡¯s 2012 Scientific Integrity 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 request 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.