Kirstjen Nielsen Resigning as DHS Secretary

Homeland Security Secretary Kirstjen Nielsen resigned on Sunday amid the administration's growing frustration and bitterness over the number of Central American families crossing the southern border, two people familiar with the decision said.

President Donald Trump thanked her for her work in a tweet and announced U.S. Customs and Border Protection Commissioner Kevin McAleenan would be taking over as acting head of the department. McAleenan is a longtime border official who is well-respected by members of Congress and within the administration. The decision to name an immigration officer to the post reflects Trump's priority for a sprawling department founded to combat terrorism following the Sept. 11 attacks.

Though Trump aides were eyeing a staff shake-up at Homeland Security and had already withdrawn the nomination for another key immigration post, the development Sunday was unexpected.

Nielsen traveled to the U.S.-Mexico border on Friday with Trump to participate in a roundtable with border officers and local law enforcement. There she echoed Trump's comments on the situation at the border, though she ducked out of the room without explanation for some time while Trump spoke. As they toured a section of newly rebuilt barriers, Nielsen was at Trump's side, introducing him to local officials. She returned to Washington afterward on a Coast Guard Gulfstream, as Trump continued on a fundraising trip to California and Nevada.

But privately, she had grown increasingly frustrated by what she saw as a lack of support from other departments and increased meddling by Trump aides, the people said. She went into a meeting with Trump at the White House in Sunday not knowing whether she'd be fired or would resign, and she ended up resigning, they said.

There have been persistent tensions between the White House and Nielsen almost from the moment she became secretary, after her predecessor John Kelly became the White House chief of staff in 2017. Nielsen was viewed as resistant to some of the harshest immigration measures supported by the president and his aides, particularly senior adviser Stephen Miller, both around the border and on other matters like protected status for some refugees. Once Kelly left the White House last year, Nielsen's days appeared to be numbered. She had expected to be pushed out last November, but her exit never materialized. And during the government shutdown over Trump's push for funding for a border wall, Nielsen's stock inside the White House even appeared to rise.

But in recent weeks, as a new wave of migration has taxed resources along the border and as Trump sought to regain control of the issue for his 2020 re-election campaign, tensions flared anew.

Nielsen, 46, has held the position since December 2017. She led a sprawling department responsible for carrying out some of Trump's most contentious policies, such as his plan to build a wall on the border with Mexico and the separation of migrant children from their families.

It was not the first time that Nielsen's supposed departure had been reported. The Wall Street Journal reported in November that Trump had decided to replace Nielsen.

The resignation comes days after the White House suddenly withdrew the nomination of Ron Vitiello, a longtime border official to lead U.S. Immigration and Customs Enforcement, to go with someone “tougher.”

"We're going in a little different direction. Ron's a good man but we're going in a tougher direction. We want to go in a tougher direction," Trump told reporters Friday at the White House.

Stephen Miller directly lobbied Trump to go in a different direction, telling him that Vitiello was not fully in favor of closing the southern border as Trump has threatened to do so in recent days.

No one in the Department of Homeland Security, though, had been notified about the intention to remove Vitiello, a sure lock for the position who has been acting head of the agency since June. Trump’s administration is struggling to deal with an influx of Central American immigrants as U.S.-Mexico border crossings are at a 12-year high and straining the U.S. immigration system.

The Washington Post's Robert Costa tweeted that Energy Secretary Rick Perry and Virginia Attorney General Ken Cuccinelli were the frontrunners to the permanent nomination.

Newsmax writer Solange Reyner and Reuters contributed to this report.