In a tweet on Sunday, President Donald Trump wrote that U.S. Customs and Border Protection (CBP) Commissioner Kevin McAleenan will become the new acting head of the Department of Homeland Security after the resignation of Secretary Kirstjen Nielsen.

Nielsen had been in the post since December 2017.

“Secretary of Homeland Security Kirstjen Nielsen will be leaving her position,” the president said, “and I would like to thank her for her service,” he said.

He added, “I am pleased to announce that Kevin McAleenan, the current U.S. Customs and Border Protection commissioner, will become acting secretary … I have confidence that Kevin will do a great job!”

Nielsen’s exit came just two days after Trump visited the border in Calexico, California, on Friday.

During that visit, he said that America’s asylum system is “full” and could not take any more individuals. He also said some of those seeking asylum at the border were gang members.

“I look at some of these asylum people, they’re gang members,” he said. “They’re not afraid of anything. They have lawyers greeting them. They read what lawyers tell them to read.” It’s a “scam,” Trump added.

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“It’s a scam, OK? It’s a hoax. I know about hoaxes,” he added. “I just went through a hoax.

Who is Kevin McAleenan and what are his credentials?

Here’s a quick rundown of key facts.

1.) McAleenan is a longtime border officer, “reflecting Trump’s priority for the department initially founded to combat terrorism after the September 11 attacks,” as Fox News noted.

2.) Trump wanted the “toughest cop” around on border security — “and McAleenan fit the bill,” the outlet also reported.

3.) McAleenan served as head of Customs and Border Patrol and was the nation’s top border security official; he was sworn into that job in March 2018. Prior to his confirmation, he was acting commissioner beginning Jan. 20, 2017, according to his biography.

4.) In that role, McAleenan oversaw 60,000 employees, managed a budget of over $13 billion, and ensured “CBP’s mission to protect national security while promoting economic prosperity,” as his biography also noted.

5.) Before that, he held several leadership positions at CBP and at one of its agencies, the U.S. Customs Service. In December 2011, he became acting assistant commissioner of CBP’s Office of Field Operations, leading “agency operations to secure the U.S. border while expediting lawful trade and travel at 329 ports of entry in the United States and 70 international locations in more than 40 countries.”

6.) From 2006 to 2008, he served as area port director of Los Angeles International Airport, directing CBP’s border security operations there and at 17 other airport facilities.

7.) After the 9/11 terror attacks, McAleenan focused on national security issues. In November 2001, he helped establish the Office of Antiterrorism in Washington, D.C. Two years later, he became executive director.

8.) Prior to government service, McAleenan practiced law in California. He received his law degree from the University of Chicago Law School.

9.) He earned a bachelor of arts in political science from Amherst College in Massachusetts.

10.) He is 47 and was born in Honolulu, Hawaii. He is married to Corina Avalos McAleenan, a Deloitte executive; they have two children.