Scott Pruitt has resigned as head of the Environmental Protection Agency, President Donald Trump announced Thursday in a tweet.

Pruitt, the former lawyer and Republican attorney general from Oklahoma, has faced at least a dozen ethics investigations since he joined the Trump administration in February 2017, including into allegations that he spent lavishly on travel, retaliated against employees and asked aides to carry out personal errands, including helping his wife secure a job.

Other expenses that have drawn scrutiny in recent months include more than $3 million for a 24-hour security team of 20 armed officers and tens of thousands of dollars on a soundproof booth in his office.

“The unrelenting attacks on me personally, my family, are unprecedented and have taken a sizable toll on all of us.” – Scott Pruitt

In a resignation letter obtained by the PBS NewsHour, Pruitt wrote that “it is extremely difficult for me to cease serving you in this role first because I count it a blessing to be serving you in any capacity, but also, because of the transformative work that is occurring.”

“However, the unrelenting attacks on me personally, my family, are unprecedented and have taken a sizable toll on all of us,” he continued.

On Twitter, Trump wrote that “Scott has done an outstanding job, and I will always be thankful to him for this.”

Pruitt joins a short list of other Trump officials — including former Veterans Affairs Secretary David Shulkin and Health and Human Services Secretary Tom Price — to leave the administration amid ethics investigations.

Andrew Wheeler, Pruitt’s deputy and a former coal lobbyist, will assume the role of acting administrator of the EPA starting Monday.

Read former EPA chief Scott Pruitt’s full resignation letter

Pruitt, who sued the EPA more than a dozen times as Oklahoma’s attorney general, reversed or took steps to repeal a number of environmental regulations during his tenure, including the Clean Power Plan and fuel emissions standards set during the Obama administration — part of a larger effort to downsize the agency, which he and other critics have described as inflated and overreaching.

He also supported Trump’s decision to withdraw from the Paris Climate agreement, continuing his public questioning of climate change and the impact of greenhouse gas emissions.

Pruitt has reduced the EPA’s budget and helped roll back at least 24 environmental regulations since taking office, according to the Harvard Law School’s Environmental and Energy Law Program, which has tracked such rollbacks under Trump.

READ MORE: All of the ways embattled EPA chief Scott Pruitt has changed energy policy

In a statement to local radio station KRMG, U.S. Sen. Jim Inhofe, R-Okla., said Pruitt “did great work to reduce the nations regulatory burdens facing our nation while leading the Environmental Protection Agency. He was single minded at restoring the EPA to its proper statutory authority and ending the burdensome regulations that have stifled economic growth across the country.”

Melina Pierce, legislative director of the Sierra Club, told the PBS NewsHour earlier this year that Pruitt “went in to undermine the EPA,” echoing other advocacy groups who felt he was harming efforts to protect land and clean water, among other environmental issues. “The intimidation and retaliation, the reassignments, the desire to cut staffing, that has all had an immediate and potentially lasting impact on the talent and expertise of the agency.”

White House officials said prior to Pruitt’s resignation that they were concerned about replacing him, Jennifer Dlouhy of Bloomberg told the NewsHour’s William Brangham on Wednesday.

“Potential successors who can be confirmed by the narrowly divided Senate are unlikely to bring the same kind of enthusiasm to reining in the EPA’s authority or rolling back these rules,” Dlouhy said.

“Potential successors who can be confirmed by the narrowly divided Senate are unlikely to bring the same kind of enthusiasm to reining in the EPA’s authority or rolling back these rules.”

Washington Post reporter Juliet Eilperin told the NewsHour’s Judy Woodruff on Thursday that “there’s no question [Wheeler is] equally committed to the policy agenda that Scott Pruitt has pursued” or that the “EPA will continue on the same policy trajectory,” though she added that it would take several months before the impact of that agenda became clear.

Environmental and transparency groups cheered Pruitt’s resignation, including Ken Cook, the president of the Environmental Working Group, who said in a statement that “after 18 months of ethical scandals, profligate spending of taxpayer dollars and rollbacks of public health protections … Scott Pruitt will go down in history as a disgrace to the office of EPA administrator.”

Pruitt “used taxpayer dollars to benefit himself while working to gut Lake Erie cleanup and the Renewable Fuel Standard and the Ohio jobs that depend on them. He never should have been confirmed in the first place, and it’s past time for him to go,” Sen. Sherrod Brown, D-Ohio, wrote on Twitter.

Danielle Brian, executive director of the Project On Government Oversight, said in a statement that “any open Inspector General investigation into Pruitt’s actions should be completed, to determine whether there should be changes in the laws or regulations governing his former position.”

She added that she hoped Trump would “choose a candidate for EPA Administrator who can rebuild—and maintain—the public’s confidence.”