(NewsNation) — U.S. Customs and Border Protection Commissioner Chris Magnus resigned Saturday, the White House announced.

“It has been a privilege and honor to be part of your administration,” Magnus wrote in a letter to the president.

Politico reported Homeland Security Secretary Alejandro Mayorkas told Magnus on Wednesday that he should either resign or be dismissed, a day after Tuesday’s midterm elections.

As the former head of the U.S. Customs and Border Protection agency, Magnus has reportedly clashed with immigration officials over how to handle migrants at the US-Mexico border.

“President Biden appreciates Commissioner Magnus’ nearly forty years of service and the contributions he made to police reform during his tenure as police chief in three U.S. cities,” press secretary Karine Jean-Pierre said in a statement. “The President thanks Mr. Magnus for his service at CBP and wishes him well.”

Before Saturday, Magnus had indicated that he refused to step down. In a statement to the Los Angeles Times, he defended his record.

“I expressed to him (Mayorkas) that I felt there was no justification for me to resign when I still cared deeply about the work I was doing and felt that that work was focused on the things I was hired to do in the first place,” Magnus said.

Magnus has served as head of the CBP since December, after previously being police chief in Tucson, Arizona, Fargo, North Dakota, and Richmond, California.

The L.A. Times said he has been vocal on immigration issues and has written on Twitter that Title 42, a Trump-era policy enacted during COVID-19 allowing Customs and Border Patrol agents to turn away migrants, “comes at a heavy cost to many asylum seekers.”

Magnus has tried changing the agency’s culture, according to the Times, with goals such as adding more female border patrol officers, and tightening rules for pursuing migrants fleeing in cars.

Magnus’ removal, the Associated Press reports, is part of a larger shakeup expected at Homeland Security as it struggles to manage migrants coming from a wide range of countries.

Migrants, according to the AP, were stopped 2.38 million times at the Mexican border in the fiscal year that ended Sept. 30. That’s up 37% from the previous year.

This story is developing. Refresh for updates.

The Associated Press contributed to this report.