Scott Pruitt, the controversial chief of the United States’ Environmental Protection Agency, has resigned. Announcing the resignation, United States President Donald Trump wrote on Twitter on Friday, “Within the agency, Scott has done an outstanding job, and I will always be thankful to him for this.”

Pruitt’s deputy at the Environmental Protection Agency, Andrew Wheeler, will take over as the acting administrator from Monday. “I have no doubt that Andy will continue on with our great and lasting EPA agenda,” Trump wrote. “We have made tremendous progress and the future of the EPA is very bright!”

As of June, Pruitt was the subject of at least 14 federal investigations, according to The Guardian. He is accused of using public funds for personal travel or office improvements, bypassing the White House to secure huge salary increases for some of his staff members, renting an apartment with ties to a lobbyist from the energy industry and several other improprieties at the Environmental Protection Agency.

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He has also angered several environmentalists by curtailing the agency’s activities and reversing decisions that protected the environment.

However, Pruitt struck an unapologetic tone in his resignation letter. “It is extremely difficult for me to cease serving you in this role,” Pruitt wrote. “However, the unrelenting attacks on me personally, my family, are unprecedented and have taken a sizable toll on all of us.”

He is the fourth Cabinet-level official of the Donald Trump administration to either resign or be fired, following Health and Human Services Secretary Tom Price, Secretary of State Rex Tillerson and Veteran Affairs Secretary David Shulkin.

Others who have either resigned or been sacked in recent months include Chief Economic Adviser Gary Cohn and National Security Advisor HR McMaster. National Security Advisor Michael Flynn resigned in February 2017, White House Chief of Staff Reince Priebus in July 2017, and Chief Strategist Steve Bannon in August.