WASHINGTON (AP) — President Donald Trump on Thursday fired his embattled Homeland Security Secretary Kristi Noem and said he will nominate in her place Oklahoma Republican Sen. Markwayne Mullin.

Trump made the announcement on social media on Thursday, two days after Noem faced a grilling on Capitol Hill from GOP members as well as Democrats.

Trump says he’ll make Noem a “Special Envoy for The Shield of the Americas,” a new security initiative that he said would focus on the Western Hemisphere.

Noem is the first Cabinet secretary to leave during Trump’s second term. Noem’s departure caps a tumultuous tenure overseeing immigration enforcement tactics that have been met with protests and lawsuits.

Noem has faced waves of criticism as she’s overseen Trump’s immigration crackdown, especially since the shooting deaths of two protesters in Minneapolis at the hands of immigration enforcement officers. The former South Dakota governor was also criticized over the way her department has spent billions of dollars allocated to it by Congress.

Frustrations over Noem’s execution of the Republican president’s hard-line immigration agenda — particularly her leadership after the shooting deaths of the two U.S. citizens in Minneapolis — as well as her handling of disaster response, paved the way for her downfall. She faced blistering criticism from Democrats, and some Republicans, in Congress hearings this week over those issues and others.

Aside from immigration, Noem also faced criticism — including from Republicans — over the pace of emergency funding approved through the Federal Emergency Management Agency and for the Trump administration’s response to disasters.

Mullin would need to be confirmed by the Senate, but under a federal law governing executive branch vacancies, he would be allowed to serve as an acting Homeland Security secretary as long as his nomination is formally pending.