Kirstjen Nielsen will step down as secretary of Homeland Security, President Donald Trump revealed on Sunday.

The development comes after Trump threatened to close the U.S.-Mexico border. The president ultimately backed off this threat after receiving warnings from advisers, business pundits and even some Republicans that such a move would hurt trade relations between the two countries and the American economy overall. Trump added U.S. Customs and Border Protection Commissioner Kevin McAleenan would replace Nielsen on an interim basis.

“Secretary of Homeland Security Kirstjen Nielsen will be leaving her position, and I would like to thank her for her service,” Trump wrote on Twitter. “I am pleased to announce that Kevin McAleenan, the current U.S. Customs and Border Protection Commissioner, will become Acting Secretary for @DHSgov. I have confidence that Kevin will do a great job!“

The number of migrants arrested at the border has risen in recent months. The Trump administration has enacted several controversial immigration policies over the past two years, including separating migrant families at the border and holding children in detention facilities. Nielsen, 46, oversaw this “zero-tolerance” policy on illegal immigration. She was also further scrutinized for her record late last after large “caravans” of Latin American migrants approached the southern border.

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The White House also on Friday withdrew Ronald Vitiello‘s nomination for director of Immigration and Customs Enforcement. Vitiello, a Border Patrol guard, had been awaiting confirmation by the Senate.

The Trump Administration has also pressured Congress to impose stricter standards for asylum and facilitate deportation of immigrants from Honduras, Guatemala and El Salvador. Trump also recently cut billions of dollars in aid to three Central American nations.

Among the people speculated to permanently replace Nielsen as head of Homeland Security are Energy Secretary Rick Perry and former Virginia Attorney General Ken Cuccinelli.