Kevin K. McAleenan, the commissioner of United States Customs and Border Protection, will become the acting secretary of the Department of Homeland Security, President Trump said in a tweet on Sunday.
The announcement came shortly after Mr. Trump said Kirstjen Nielsen was leaving the position, ending a tumultuous tenure in charge of the agency, which oversees border security, among other duties. She had taken the job in late 2017.
Mr. McAleenan, 47, will oversee an agency that has at times been a target of the president’s dissatisfaction over an increase of migrants illegally entering the United States at the southwestern border.
“I have confidence that Kevin will do a great job!” Mr. Trump said in his tweet.
In her resignation letter, Ms. Nielsen said she hoped the incoming secretary would “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.”
Mr. McAleenan has a law degree from the University of Chicago and practiced law in California before he worked for the government. He held a variety of positions at the border protection agency before becoming its deputy commissioner in 2014 under the Obama administration. He won the country’s highest civil service honor, the Presidential Rank Award, in 2015.
Mr. McAleenan became the acting commissioner of the agency in January 2017 and was formally sworn in as commissioner in March 2018. He focused on counterterrorism, border security and trade enforcement, oversaw about 60,000 employees and managed a budget of over $13 billion, according to the agency.
As the nation’s top border enforcement officer, Mr. McAleenan also helped implement the Trump administration’s “zero tolerance” policy to prosecute parents caught crossing the border illegally, which led to family separations. That policy was reversed, but the effects remain. In court documents filed on Friday, the government said it may take federal officials two years to identify what could be thousands of immigrant children who were separated from their families at the border.
In an interview in August, Mr. McAleenan said that “a better system would allow us to keep families together for the entirety of the immigration proceeding, which takes an average of 45 days.”
He added that there should be a sanction for crossing the border illegally.
“It’s not a victimless crime,” he said. “Not only are the people at risk, so are our agents who need to apprehend or rescue them. It’s also a diversionary tactic for smugglers who use family groups to tie down our agents while drugs are moved behind them.”
Mr. McAleenan said his job as the agency’s commissioner was to enforce laws, not make them.
Last month, Mr. McAleenan was the public face of new government data showing that unauthorized entries at the southwestern border were reaching record-breaking numbers; more than 76,000 migrants crossed the border without authorization in February, an 11-year high.
“The system is well beyond capacity, and remains at the breaking point,” he told reporters, adding that “this is clearly both a border security and a humanitarian crisis.”
At a Senate hearing last month, he suggested ways to address that crisis: The United States government should support governments in Central America to improve economic opportunities there, he said, and work with the Mexican authorities to clamp down on transnational criminal organizations that prey on migrants.
“And we must invest in border security,” he said. “A modern border barrier system, additional agents and officers, additional technology at and between ports of entry, and air and marine support. And we must also have a whole-of-U.S.-government approach to address children’s unique medical needs and the undeniable humanitarian challenges we are facing.”