WASHINGTON, D.C. — Scott Pruitt, the embattled leader of the U.S. Environmental Protection Agency (EPA), stepped down from his post on Friday.

Pruitt has been the subject of multiple ethics investigations. President Trump announced on Twitter on Thursday that he had accepted Pruitt’s resignation, but he continued to defend his work at the EPA.

“Within the Agency Scott has done an outstanding job, and I will always be thankful to him for this,” Trump said on Twitter.

The EPA impacts the school bus industry primarily with regulations on engine emissions and by providing funds to replace older buses and other vehicles through the Diesel Emissions Reduction Act (DERA) program.

In early 2017, there had been uncertainty about the fate of the DERA program in the Trump administration’s budgeting. The DERA funding had reportedly been one target of proposed cuts. In March 2017, a coalition of groups advocated for the program in a letter to Pruitt.

A month later, the EPA announced a new round of DERA funding for clean diesel projects. The program has continued to be funded by Congress, where it enjoys bipartisan support, and the EPA has continued to award grants and rebates. For example, in April of this year, the agency announced the availability of about $40 million in DERA grants.

Pruitt, a former Oklahoma state senator and attorney general, served as EPA administrator since February 2017. Trump had announced his intent to nominate Pruitt for the Cabinet position in December 2016, a month before Trump’s inauguration as president.

At the time, Pruitt said in a statement that “The American people are tired of seeing billions of dollars drained from our economy due to unnecessary EPA regulations, and I intend to run this agency in a way that fosters both responsible protection of the environment and freedom for American businesses.”

During his tenure, Pruitt came under fire for alleged ethics violations, with accusations including use of first-class travel and chartered jets, exorbitant office and security expenses, and housing arrangements with a lobbyist couple.

In his resignation letter to Trump, Pruitt said that “the unrelenting attacks on me personally, my family, have taken a sizable toll on all of us.”

With Pruitt’s departure, EPA Deputy Administrator Andrew Wheeler will step into the role of acting administrator on Monday. Wheeler, a lawyer and former lobbyist, was confirmed by the Senate as deputy administrator of EPA in April.

“I have no doubt that Andy will continue on with our great and lasting EPA agenda,” Trump said on Twitter.