"White House senior adviser Stephen Miller wants to make sure that outgoing Homeland Security Secretary Kirstjen Nielsen is only the first of a string of senior officials headed out the door...."

"... as he is given more authority over immigration policy. The President has also pushed in recent weeks to reinstate the family separation policy, which Nielsen resisted.... Miller's heightened influence within the West Wing has been aided by the President, who recently told aides in an Oval Office meeting that Miller was in charge of all immigration and border related issues in the White House, according to a person familiar with the meeting. Trump administration officials say that Miller, who played key a role in Nielsen's ouster, wants the President to dismiss the director of United States Citizenship and Immigration Services, Lee Cissna, and the department's general counsel, John Mitnick. United States Secret Service Director Randolph 'Tex' Alles, is also being removed from his position... 'There is a near-systematic purge happening at the nation's second-largest national security agency'...."

From "Stephen Miller wants Trump to oust more senior leaders at Homeland Security" (CNN).