Kirstjen Nielsen's abrupt departure as secretary of the Department of Homeland Security (DHS) comes as President Trump seeks to steer his administration towards a "tougher" approach on immigration to confront a surge in migrant families heading to the U.S.
The president said Kevin McAleenan, who now serves as the commissioner of U.S. Customs and Border Protection, would serve as acting secretary of the department of Homeland Security in Nielsen's absence.
She added that "our country and the men and women of DHS deserve to have all the tools and resources they need to execute the mission entrusted to them".
This is a developing story. The Wall Street Journal reported in November that Trump had chose to replace Nielsen. Nielsen had recently warned a congressional panel of a "catastrophe" on the Southern border after the number of crossings hit a 10-year high.
"Hampered by misstep after misstep, Kirstjen Nielsen's tenure at the Department of Homeland Security was a disaster from the start".
She didn't then, and went on to become one of the public faces of a controversial policy that separated undocumented children from their parents.More news: Aussies face uphill battle at Hong Kong 7s
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Nielsen has spearheaded the administration's efforts to crack down on illegal immigration, and joined President Trump in a visit to California to inspect the newly built sections of the U.S. -Mexico border fence over the weekend.
Trump recently threatened to close the border with Mexico, or parts of it, if Congress did not change USA laws to fix what he called immigration "loopholes".
That said, she was prepared to be fired at any moment, but did not know going into Sunday's meeting it would be imminent.
She became a central figure in the backlash to the Trump administration's family separation policy last summer. The Wall Street Journal reported in November that Trump had chose to replace Ms Nielsen.
It was not the first time Ms Nielsen's supposed departure has been reported.
Her replacement, McAleenan, has helped shape numerous administration's policies to date and is considered highly competent by congressional leaders, the White House and Homeland Security officials.