Two sources said Nielsen went to the White House Sunday to speak with Trump following their trip to the border late last week. The people say she has been frustrated by difficulty getting other departments to help with the growing number of families coming crossing the border.
Nielsen had been on the chopping block before. Trump threatened to fire her and she previously considered resigning, but officials at the time recognized there were no obvious successors in place.
The people were not authorized to speak publicly about internal matters and spoke to The AP on condition of anonymity.
He added that U.S. Customs and Border Protection Commissioner Kevin McAleenan will become Acting Secretary.