More top Department of Homeland Security (DHS) officials are leaving the Trump administration, two senior administration officials said Monday, increasing the leadership vacuum at DHS as Washington grapples with the news of DHS Secretary Kirstjen Nielsen's resignation.

U.S. Secret Service Director Randolph "Tex" Alles was fired Monday, and U.S. Citizenship and Immigration Services Director Lee Cissna, DHS undersecretary for management Claire Grady and DHS general counsel John Mitnick are also leaving the administration. The officials' departures are part of a system-wide purge of senior management at DHS as top Trump adviser Stephen Miller looks to overhaul immigration-related policy and personnel.

Alles learned from the White House that he would be part of a mass overhaul of DHS leadership a few weeks ago. The notification came prior to the security incident at Mar-a-Lago on March 30, a law enforcement official told CBS News. White House press secretary Sarah Sanders confirmed in a statement that Alles will leave "shortly" and James M. Murray, a career Secret Service member, will take over in May. One Secret Service source described Murray as a "good pick."

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"United States Secret Service director Randolph 'Tex' Alles has done a great job at the agency over the last two years, and the president is thankful for his over 40 years of service to the country," Sanders said. "Mr. Alles will be leaving shortly and President Trump has selected James M. Murray, a career member of the USSS, to take over as director beginning in May."

Alles' wife, Sarah Alles, told reporters, "We're very proud of the Secret Service. The agents are very professional, so pleased he was able to serve. They serve the nation well."

Grady was the official standing in the way in December when the president wanted to oust Nielsen. The president would have had to place Grady in charge of DHS, given that she was the most senior Senate-confirmed official there.

A source on the House Homeland Security Committee says Chairman Bennie Thompson, whose responsibility it is to oversee DHS, was given no heads up on the departures. It's unclear if the White House gave Senate Homeland Security Committee Chairman Ron Johnson, a Republican, any heads up.

These departures also come as Mr. Trump announced last week he wants to go in a "tougher" direction than his original pick for Immigration and Customs Enforcement, Tom Vitiello.

Mr. Trump has repeatedly expressed his frustration with the flow of migrants crossing the U.S.-Mexico border, and his top officials' inability to fix the matter. In recent days, he has threatened to close the border, or at least large parts of it, if he doesn't get what he wants.

Arden Farhi and Kathryn Watson contributed to this report.