WASHINGTON — President Donald Trump fired his embattled homeland security secretary, Kristi Noem, on Thursday and announced plans to replace her with Sen. Markwayne Mullin of Oklahoma, after she was grilled by Republican lawmakers this week at congressional hearings on a variety of topics, including her knowledge of a contract with a politically connected firm.

Trump announced the change on social media, along with a new, and previously nonexistent, role for Noem: special envoy for the Shield of the Americas, which he said would be a new security initiative for the Western Hemisphere.

Trump is close with Mullin, a Republican, and speaks with him regularly. Mullin said that Noem had “done the best that she could do under the circumstances,” but that he hoped to learn from her tenure and “build off things that didn’t quite go as planned.”

Noem, the first Cabinet member to be ousted in Trump’s second term, had been among the key figures in the administration fulfilling his mass deportation effort, which he campaigned on aggressively and which was heavily influenced by Stephen Miller, a top White House adviser.

But her tenure had been marked by a string of controversies, and her fate had been the focus of speculation among Trump’s allies for several weeks. On Thursday, the president contradicted remarks that Noem made under penalty of perjury in her hearing before a Senate panel on Tuesday: that Trump had signed off on a border security advertising campaign featuring Noem.

“I never knew anything about it,” Trump told Reuters. A White House spokesperson declined to comment and referred a reporter to Trump’s comments to Reuters.

Noem has faced scrutiny from lawmakers about the campaign, on which the government spent $220 million.

The ads prominently featured Noem, including in a scene filmed on horseback at Mount Rushmore in the former South Dakota governor’s home state.

Pressed at a separate hearing on Tuesday about the process for awarding the contracts behind the ad campaign, Noem said that it all went through “a competitive process,” and that no political appointees were involved. On Wednesday, she said the contract was “all done correctly, all done legally.”

Sen. John Kennedy, R-La., forcefully questioned Noem over ad contracts. Kennedy told reporters on Thursday that he received a call from the president about her testimony. “Put it this way,” he said. “His recollection and her recollection are different.”

Kennedy told reporters on Tuesday that he had spoken with White House officials ahead of Tuesday’s hearing to warn them that he would be asking Noem some tough questions.

During the hearing, Kennedy repeatedly pressed Noem over a ProPublica report that found that the Strategy Group, a firm based in Ohio, had been paid in connection with the campaign. The firm is led by Ben Yoho, who is married to Tricia McLaughlin, Noem’s former spokesperson, who recently left the department.

The firm posted on social media on Wednesday that it had a subcontract with a firm that had a contract with the department, and that it was a different entity that held the larger contract.

Noem defended the campaign as important to Trump’s immigration agenda.

“The president tasked me with getting the message out to the country,” she told Kennedy.

Skeptical, Kennedy repeatedly asked Noem whether Trump signed off before the ad campaign. She replied, “We had that conversation, yes, before I was put in this position and sworn in and confirmed. And since then as well.” She then reiterated that the advertisements had been “extremely effective.”

Kennedy, who has not been among Noem’s most vocal critics, shot back, “Well, they were effective in your name recognition.” Then he added that she had put Trump “in a terribly awkward spot.”

At another point, Kennedy asked her about her unfounded claims that Alex Pretti and Renee Good — the two U.S. citizens killed by federal immigration agents in Minneapolis — had committed acts of domestic terrorism.

Citing an Axios report, Kennedy accused Noem of blaming those comments on Miller. “Do you think it was fair to blame Mr. Miller for your words?” he asked. Noem dismissed the report as false. (Miller, in posts on the social site X, called Pretti “an assassin” and “a domestic terrorist.”)

Noem’s tenure was filled with controversy. From the beginning, she was aggressive in becoming the public messenger of the president’s mass deportation campaign. She starred in ads urging immigrants to leave the country, appeared on television regularly and even did a video inside of a Salvadoran prison holding Venezuelan migrants who had been shipped out under wartime powers. Some inside her department felt Noem was using her perch as secretary to advance her political ambitions.

Noem’s response to the shooting of Pretti in January, perhaps more than any other moment during her tenure, invited questions about whether she was up to the task of running the department. After the shooting, Noem immediately called Pretti a domestic terrorist, and even alleged that he was brandishing a gun, a charge that was later undermined by an initial review of the incident.

Noem met with Trump in the Oval Office after the incident, and he continued to support her.