A key architect of the Trump administration’s efforts to weaken federal climate rules is under scrutiny by a federal watchdog for his dealings with industry players who lobbied the government to ease carbon pollution limits. It is the third inquiry into whether Bill Wehrum, who headed the Environmental Protection Agency’s air policy division from November 2017 until last month, violated federal ethics rules.

Scott Pruitt, the administrator of the Environmental Protection Agency and architect of President Trump’s aggressive effort to rewrite the government’s rule book on environmental  regulations, resigned on Thursday in the face of numerous ethics investigations that doomed his tenure. Despite Mr. Pruitt’s efforts to nurture a close relationship with the president, Mr. Trump himself announced the resignation in a tweet sent from Air Force One. He thanked Mr. Pruitt for an “outstanding job” and said the agency’s deputy, Andrew Wheeler, a former coal lobbyist, would take over as the acting administrator on Monday.

Just one day after Environmental Protection Agency (EPA) Administrator Scott Pruitt told lawmakers on Capitol Hill that the agency did not have plans “at present” to target California’s special fuel economy waiver, the Trump administration announced that it would be putting a six-year hold on Obama-era standards for fuel efficiency, starting in 2020, while at the same time revoking California’s waiver to set its own standards.

Scott Pruitt, the head of the Environmental Protection Agency, rented a residence in Washington in 2017 that was partly owned by the wife of a top energy lobbyist whose firm, according to disclosure forms, conducted business before the E.P.A. that same year.