Department of Homeland Security (DHS) Secretary Kirstjen Nielsen suddenly resigned on Sunday night. Her last day in the department will be on Wednesday and she promised a smooth transition.

Nielsen’s resignation comes as President Donald Trump becomes becoming more frustrated with the continuous influx of large number of Central American migrant families entering the southern border and seeking asylum.

Media reports indicated that she resigned due to her differences with the President and the increased meddling of his aides over DHS authorities on matters such as providing protected status for some refugees. She reportedly resisted resuming a policy that resulted to family separation at the border.

In her resignation letter, Nielsen said it was her “great honor to lead the men and women” of the DHS. According to her, “I could not be prouder and more humbled in their service, dedication, and commitment to keep our country safe from all threats and hazards.”

Additionally, Nielsen stated that she “worked tirelessly to advance the goals and missions of the Department” during her tenure. She is very proud with their achievements in transforming the DHS to keep pace with the country’s enemies whether in cyberspace or emerging threats from new technologies

“Despite our progress in reforming homeland security for a new age, I have determined that it is the right time for me to step aside…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,” wrote Nielsen.

Trump appoints CBP Commissioner McAleenan as Acting DHS Secretary

In a tweet, Trump thanked Nielsen for her service. He also announced that U.S. Customs and Border Protection (CBP) Commissioner Kevin McAleenan will serve as Acting DHS Secretary.

On Monday, Nielsen told reporters, “As you know, DHS has a vast array of missions. I want to make sure that we execute them all with excellence through the transition. I share the president's goal of securing the border."

The outgoing DHS Secretary said she’s been “talking with government officials, administration officials, members of Congress to ensure a smooth transition."