WASHINGTON (AP) — Kirstjen Nielsen said Monday she still shares President Donald Trump’s goal of securing the Mexican border, a day after she resigned as Homeland Security secretary amid Trump’s frustration and bitterness over a spike in Central American migration.

At the same time, the White House announced Monday that Trump was replacing Secret Service Director Randolph “Tex” Alles, the latest development in the shake-up in the upper echelon of Nielsen’s department. Alles’ departure, to be replaced by career Secret Service official James Murray, is unrelated to Nielsen’s resignation, according to administration officials, who spoke only on condition of anonymity to discuss the personnel matter. Yet it is part of a DHS turnover that began last week when Trump withdrew his Immigration and Customs Enforcement director’s nomination to stay on permanently.

Nielsen had 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 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 as Trump continued on a fundraising trip to California and Nevada.

But on Sunday, she wrote in her resignation letter that “it is the right time for me to step aside.” She wrote that she hoped “the next secretary will have the support of Congress and the courts in fixing the laws which have impeded our ability to fully secure America’s borders and which have contributed to discord in our nation’s discourse.”

Trump announced on Sunday in a tweet that U.S. Customs and Border Protection Commissioner Kevin McAleenan would be taking over as acting head of the department. The decision to name a top immigration officer to the post reflects Trump’s priority for the sprawling department founded to combat terrorism following the Sept. 11, 2001, attacks.

On Monday, Nielsen told reporters outside her Alexandria, Virginia, home that she continues to support the president’s goal of securing the border.

“I will continue to support all efforts to address the humanitarian and security crisis on the border,” she said in her first public remarks since the surprise resignation, thanking the president “for the tremendous opportunity to serve this country.”

Yet Nielsen had grown increasingly frustrated by what she saw as a lack of support from other departments and increased meddling by Trump aides on difficult immigration issues, according to three people familiar with details of her resignation. They spoke on condition of anonymity because they were not authorized to discuss the matter.

She went to the White House on Sunday to meet with Trump not knowing whether she’d be fired or would resign. She ended up resigning, though she was not forced to do so, they said.

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

Still, it was unclear how McAleenan would immediately assume the role. The agency’s undersecretary of management, Claire Grady, is technically next in line for the job, and she will need to resign — or more likely be fired — in order for McAleenan to assume the post.

White House spokesman Hogan Gidley spoke of McAleenan’s “extensive” knowledge of immigration issues and said the change in leadership would hopefully lead to “massive changes” at the border.

Nielsen is the latest person felled in the Trump administration’s unprecedented churn of top staff and Cabinet officials, brought about by the president’s management style, insistence on blind loyalty and rash policy announcements.

She was also the highest profile female Cabinet member, and her exit leaves DHS along with the Pentagon and even the White House staff without permanent heads. Patrick Shanahan has held the post of acting defense secretary since Jim Mattis was pushed out in December over criticism of the president’s Syria withdrawal plans. Acting White House chief of staff Mick Mulvaney has held his post since January, following John Kelly’s resignation last year.

McAleenan has helped shape many of the administration’s policies to date and is considered highly competent by congressional leaders, the White House and Homeland Security officials. But it’s unclear if he can have much more of an effect on the issues at the border. The Trump administration has bumped up against legal restrictions and court rulings that have hamstrung many of its major efforts to remake border security.

Rep. Joaquin Castro, the Texas Democrat who is chairman of the Congressional Hispanic Caucus, was critical of Nielsen, saying she spent her tenure “championing President Trump’s cruel anti-immigrant agenda,” and he called McAleenan’s appointment “deeply disturbing … based on his record of prioritizing Trump’s harmful policies.”

Tensions between the White House and Nielsen have persisted almost from the moment she became secretary, after her predecessor, 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 on matters around the border and others like protected status for some refugees.

The final straw came when Trump gave Nielsen no heads-up or opportunity to discuss his decision to withdraw the nomination of acting Immigration and Customs Enforcement Director Ron Vitiello — a move seen as part of a larger effort by Miller, an immigration hardliner, and his allies at the White House to clean house at the department.

Arrests along the southern border have skyrocketed recently. Border agents are on track to make 100,000 arrests and denials of entry at the southern border in March, more than half of which are families with children. A press conference to announce the most recent border numbers — scheduled to be held by McAleenan on Monday — was postponed.

Nielsen dutifully pushed Trump’s immigration policies, including funding for his border wall, and defended the administration’s practice of separating children from parents. She told a Senate committee that removing children from parents facing criminal charges happens “in the United States every day.” But she was also instrumental in ending the policy.

Under Nielsen, migrants seeking asylum are waiting in Mexico as their cases progress. She also moved to abandon longstanding regulations that dictate how long children are allowed to be held in immigration detention and requested bed space from the U.S. military for 12,000 people in an effort to detain all families who cross the border. Right now there is space for about 3,000 families, and facilities are at capacity.

Nielsen also advocated for strong cybersecurity defense and said she believed the next major terror attack would occur online — not by planes or bombs. She was tasked with helping states secure elections following Russian interference during the 2018 election.

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Associated Press writers Jill Colvin and Lisa Mascaro contributed to this report.

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