A top White House energy adviser said he resigned after being told he wouldn’t receive a permanent security clearance, amid intensifying scrutiny of administration officials only temporarily authorized to review classified information.

George David Banks served on the National Economic Council as a special assistant to the president for international energy and environmental policy since February 2017. The role put him on the front lines of debates about the Paris climate accord and the construction of coal plants around the globe.

Banks said he resigned after being told he wouldn’t be granted a security clearance because of marijuana use five years ago.

Banks’s move follows the high-profile resignation of two top administration aides amid questions about how many White House employees are operating on temporary security clearances and the thoroughness of that vetting. White House Staff Secretary Rob Porter resigned last week amid claims of abuse by two former wives and an ex-girlfriend, followed by White House speechwriter David Sorensen.

A White House spokeswoman didn’t respond to repeated emails about Banks’s departure, which was first reported by Politico.

Banks, a former State Department official, CIA analyst and White House adviser under former President George W. Bush, joined the Trump administration after serving as executive vice president of the pro-business American Council for Capital Formation.

At the White House, Banks was a leading advocate for the U.S. remaining in the Paris agreement, an international accord in which nearly 200 countries agreed to cut carbon dioxide emissions tied to climate change.

After President Donald Trump said the U.S. would abandon the agreement in June, Banks argued that high-efficiency coal plants could be part of the climate solution; he was the lead administration official discussing the issue at a U.S. event on the sidelines of the United Nations climate conference in Germany last November.