Environmental Protection Agency Administrator Scott Pruitt has been cleared of an environmental group's accusations that he violated the agency's scientific integrity policy by saying that carbon dioxide was not the principal cause of global warming.

The agency's Office of the Science Adviser said Pruitt has the right to express his opinion even if it is found to be controversial, according to a Tuesday report.

"The freedom to express one's opinion about science is fundamental to EPA's Scientific Integrity Policy even [and especially] when that point of view might be controversial," said Thomas Sinks, the office's director.

The decision follows an investigation that was started after a petition from the Sierra Club was filed in March claiming that Pruitt violated EPA's rules of scientific integrity that the office is obligated to consider and investigate.

The Sierra Club referred to Pruitt as a "climate denier" in its complaint. The EPA board that reviewed and denied the complaint admonished the environmental group over its interpretation of the agency's scientific integrity policy, according to the Washington Free Beacon, which first reported the findings after obtaining a letter from the board.

"Expressing an opinion about science is not a violation of the EPA Scientific Integrity Policy," the board said. "Indeed, the Scientific Integrity Policy — in the spirit of promoting vigorous debate and inquiry — specifically encourages employees to express their opinion should the employee disagree with scientific data, scientific interpretations or scientific conclusions."