U.S. Homeland Security Secretary Kirstjen Nielsen resigned Sunday amid President Donald Trump’s growing frustration and bitterness over the number of Central American families crossing the southern border.

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Though Trump aides were eyeing a staff shake-up at Homeland Security and had already withdrawn the nomination for another key immigration post, the development Sunday was unexpected.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

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 into 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.

(SD-Agencies)