Homeland Security Secretary Kirstjen Nielsen listens to President Donald Trump at a roundtable on immigration and border security at the U.S. Border Patrol Calexico Station in Calexico, Calif., Friday April 5, 2019. Trump headed to the border with Mexico to make a renewed push for border security as a central campaign issue for his 2020 re-election. (AP Photo/Jacquelyn Martin)

WASHINGTON - The Latest on the departure of Homeland Security Secretary Kirstjen Nielsen (all times local):

8:30 p.m.

The White House push for a harder line on immigration, including the shake-up at the Department of Homeland Security, is being orchestrated by Trump adviser Stephen Miller, the impetus behind some of the administration's most controversial policies.

That's according to three officials who spoke on condition of anonymity because they were not authorized to speak publicly. They say President Donald Trump is also considering separating migrant families at the border again, resuming the practice that drew so much outrage last year.

The officials say the head of U.S. Citizenship and Immigration Services, L. Francis Cissna, and Homeland Security General Counsel John M. Mitnick are expected to be pushed out of their positions.

Homeland Security Secretary Kirstjen Nielsen submitted her resignation Sunday.

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12:45 p.m.

Kirstjen (KEER'-sten) Nielsen says she continues to support President Donald Trump's goal of securing the U.S.-Mexico border in her first public remarks since her surprise resignation as Homeland Security secretary.

Nielsen told reporters gathered outside her Alexandria, Virginia, home on Monday that she will keep supporting "all efforts to address the humanitarian and security crisis" on the border.

Nielsen thanks the Republican president for "the tremendous opportunity to serve this country." She says she's spent the last 24 hours talking to administration officials and members of Congress to ensure a smooth transition and intends to continue "to support" the department "from the outside."

Nielsen resigned Sunday amid Trump's growing frustration and bitterness over a spike in Central American migrants crossing the border.

Customs and Border Protection Commissioner Kevin McAleenan has taken over as department head.

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12:55 a.m.

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

Trump announced on Sunday in a tweet that U.S. 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 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.

News from © The Associated Press, 2019