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Though Trump aides were eyeing a staff shake-up at Homeland Security and had already withdrawn the nomination for another key immigration post, the development Sunday was unexpected.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

Her resignation letter , obtained by The Associated Press, had not a whiff of controversy unlike others who have left from the administration.

"Despite our progress in reforming homeland security for a new age, I have determined that it is the right time for me to step aside," she wrote. "I hope that 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."

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 on matters around the border and others 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 insistence 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.

Arrests all along the southern border have skyrocketed in recent months. Border agents are on track to make 100,000 arrests and denials of entry at the southern border this month, over half of which are families with children.

Nielsen advocated for strong cybersecurity defense and often 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 2016 election.

She dutifully pushed Trump's immigration policies, including funding for his border wall, and defended the administration's practice of separating children from parents, telling 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 US military for some 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.

AP