Updated at 7:14 p.m. ET

Secretary of Homeland Security Kirstjen Nielsen is leaving her post, President Trump announced Sunday as he continues to focus on restricting border crossings amid a recent surge. Nielsen had recently warned a congressional panel of a “catastrophe” on the Southern border after the number of crossings hit a 10-year high.

Trump confirmed the news in a tweet, saying, “I would like to thank her for her service.”

Trump said Kevin McAleenan, the current commissioner of U.S. Customs and Border Protection, would serve as acting secretary. The change at the top comes with other uncertainty over management at the Department of Homeland Security. The president on Friday dropped his nominee to lead Immigration and Customs Enforcement, Ronald Vitiello, telling reporters, “We want to go in a tougher direction.”

Often the primary person working to turn Trump’s harsh immigration rhetoric into policy, Nielsen has faced intense scrutiny. As recently as March, she faced tough questioning from House Democrats over the treatment of migrants, especially children, at the Southern border.

Nielsen is Trump’s second secretary of homeland security, taking over the cabinet post in December 2017 after John Kelly became White House chief of staff. Nielsen had worked as a top aide to Kelly at both DHS and the White House.

Her tenure was marked throughout by tensions with Trump, who reportedly lashed out at her several times because illegal border crossings have not been stopped. Nielsen faced more public pressure after the Trump administration enacted a policy of family separation that resulted in thousands of migrant children being separated from their parents, which the president ultimately reversed.

In her resignation letter posted on Twitter, Nielsen alluded to her rocky tenure.

“Despite our progress in reforming homeland security for a new age, I have determined that it is the right time for to step aside,” wrote Nielsen. “I hope that the next Secretary will have the support of Congress and the courts in fixing the laws that have impeded our ability to fully secure America’s borders and have contributed to discord in our nation’s discourse.

Democrats, who have been critical of Nielsen’s actions at DHS throughout her time there, said her departure showed that Trump’s immigration policy was too extreme.

“When even the most radical voices in the administration aren’t radical enough for President Trump, you know he’s completely lost touch with the American people,” said Senate minority leader Chuck Schumer, D-N.Y. [Copyright 2019 NPR]