Nielsen spoke out for the first time since announcing her resignation outside her house Monday afternoon, telling reporters there is a "humanitarian crisis" at the border and emphasizing the need to address that. Her resignation is effective Wednesday, although Nielsen didn't leave her home in Virginia until early Monday afternoon.

"I don't have any new announcements. I just want to thank the president again for the tremendous opportunity to serve this country. I'm forever grateful and proud of the men and women of DHS who work so hard every day to execute their missions and support the homeland," she told reporters outside her home on Monday, taking no questions. "... 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."

But Nielsen's tenure since she was confirmed in December 2017 has at times been rocky, with the president taking some of his frustrations over illegal immigration out on her. Questions about whether she might leave have swirled for months. But she was by the president's side on Friday in Calexico, California, as Mr. Trump pushed for a crackdown on illegal immigration and the need for a border wall.

"This afternoon I submitted my resignation to @POTUS and thanked him for the opportunity to serve in his administration. It's been an honor of a lifetime to serve with the brave men and women of @DHSgov," Nielsen wrote in her resignation letter, which she tweeted Sunday evening. "I could not be prouder of and more humbled by their service, dedication, and commitment to keep our country safe from all threats and hazards."

Nielsen's announced exit comes two days after Mr. Trump announced he wants to go in a "tougher" direction in his nomination for the Immigration and Customs Enforcement (ICE) director, after originally announcing Ron Vitiello would head ICE. Nielsen's departure also means acting heads will soon be running DHS, the Pentagon and the U.S. Department of the Interior. Nielsen has also been one of only four women serving in Cabinet-level positions in the Trump administration, the others being Transportation Secretary Elaine Chao, CIA Director Gina Haspel and Education Secretary Betsy DeVos.

One U.S. official told CBS News is it unlikely McAleenan would be nominated as Nielsen's permanent replacement. It's unclear whether he would have to resign as CBP commissioner to take the job, and whether the line of succession at DHS would even allow for such a personnel move. Those legal issues would need to be sorted out.

McAleenan has worked as CBP commissioner since the early days of Mr. Trump's administration, keeping a generally low profile. In a 2018 interview with the New York Times in the height of the concern over family separations at the border, McAleenan called Mr. Trump's attempt to halt the separations with an executive order an "important recalibration."

In recent days, Mr. Trump has threatened to shut down the U.S.-Mexico border or slap tariffs on cars made in Mexico coming into the U.S. if Mexico and Congress don't fix the situation at the border.

Nielsen became known for her vigorous defense of the "zero tolerance" policy resulting in family separations at the border, blaming Congress for a "loophole" in the laws that needs to be fixed. Nielsen claimed in a White House briefing last year that the administration was merely continuing a policy from "previous administrations" that mandates separating a child who is "in danger, there is no custodial relationship between 'family' members, or if the adult has broken the law."

"As long as illegal entry remains a criminal offense, DHS will not look the other way," Nielsen told reporters at the time.

Nielsen was allegedly scheduled to talk to the president about immigration and border issues. She had no intention of resigning but personnel changes are being pushed for in the Trump administration, CNN reported.

From CNN:

The source notes Trump's frustration with the current asylum laws, and his desire for individuals who work for the administration to just stop central American asylum seekers from entering the United States, contrary to the law.

Separately, a source close to Nielsen and an administration official said she was blindsided by the White House pulling Ron Vitiello's nomination to be director of Immigration and Customs Enforcement and has heard of other impending immigration-related personnel changes, so she asked for a meeting with Trump to get a sense of his thinking.

According to the Washington Examiner, the White House called Vitiello on Thursday night to let him know his nomination had been rescinded, meaning he wouldn't travel with Nielsen and Trump to the border on Friday.

It has been rumored that Vitiello's nomination was pulled because he was not endorsed by ICE union leader Chris Crane.

• China's Silk Roads - From the Caucasus to the Balkans, the Belt and Road Initiative is challenging the West's global domination
• China's One Belt One Road is engine of growth for Eurasia, trade volume over $5 trillion for past five years
• How does Islamic State and Tibet affect China's crackdown in Xinjiang?
• What the media isn't telling you about China's 'Xinjiang problem' - it's made in the USA

• In Memoriam: 5 Years Ago Today US and NATO Psychopaths Butchered Libya and Publicly Murdered Gaddafi
• Libyan National Army led by Haftar begins offensive on UN-backed govt in Tripoli - UPDATE
• Libyan Army launches "Volcano of Rage" military operation to defeat Haftar's coup forces

• Erdogan threatens to wipe out 'terror corridor' in Syria & Iraq
• US weapons to loyal groups in Syria triggered Turkey's military operation against Kurds
• Bolton warns Turkey against attacking Kurd fighters after US pulls out of Syria
• Russian Security Council: US gave Kurds modern arms, made Turkey launch Afrin op
• Erdogan says Turkish forces will clear Syria of Kurdish fighters 'all the way to Iraqi border'

• The US stealth war on Venezuela
• Maduro: 'Electrical power coup is in full swing' - Appoints new energy staff
• 2 perpetrators of Venezuelan power grid 'sabotage' captured, according to authorities
• Venezuela under attack: 7 notes on the US' electric shock treatment
• President Assad to visiting Venezuelan FM: 'Your crisis similar to what happened in Syria'
• Russia might be planning an Astana-like conference for Venezuela