A widening contradiction between President Donald Trump and his homeland security secretary over a $220 million government advertising campaign preceded the downfall of Kristi Noem, culminating Thursday in her removal from the post and replacement by Markwayne Mullin, a Republican senator from Oklahoma.

Trump announced the move on his social media website, Truth Social, a platform owned by Trump Media & Technology Group Corp (NASDAQ:DJT).

Mullin, Trump explained, would take over leadership of the U.S. Department of Homeland Security. At the same time, Noem would assume a newly created role as special envoy for the so-called “Shield of the Americas” — apparently a new security initiative.

The leadership shake-up followed contentious congressional hearings in which Noem defended a multimillion-dollar border security advertising campaign. Footage features the former governor of South Dakota rode on horseback near Mount Rushmore.

During sworn testimony before lawmakers, Noem said Trump approved the ad campaign. But the president publicly contradicted her account a day later.

“I never knew anything about it,” Trump told Reuters.

The advertising campaign cost taxpayers roughly $220 million, according to the New York Times. The firm that handled the ad had ties to the husband of Noem’s former spokeswoman.

Lawmakers pressed Noem about the contract during hearings this week.

Republican Senator John Kennedy of Louisiana spoke with Trump about the situation. The president’s version of the events, Kennedy said, differed from Noem’s.

Noem also recently oversaw recent controversies regarding two fatal encounters between immigration agents and U.S. citizens in Minneapolis: the shootings of Renee Nicole Good and Alex Pretti.

In the immediate aftermath of the incidents, Noem described both victims as “domestic terrorists,” defending the actions of federal immigration agents who fired the shots. The remarks sparked national backlash, particularly after video from the shootings appeared to show a situation that did not present an immediate threat to officers.

Noem has not retracted the statements or acknowledged the video evidence publicly.

Shares of Trump Media & Technology Group Corp declined 3.29% at last check on Thursday, trading at around $10.59.

Cody Scanlan/The Register / USA TODAY NETWORK via Imagn Images