U.S. Customs and Border Protection Commissioner Chris Magnus resigned on Saturday after less than a year in the role, adding another layer of upheaval to an already-tumultuous situation at the U.S.-Mexico border.

President Joe Biden accepted Magnus’s resignation letter after a brief standoff with Homeland Security Secretary Alejandro Mayorkas, according to a Washington Post report. Mayorkas had asked Magnus to step down earlier in the week, but Magnus said he would not leave until asked by the White House.

“Thank you for the opportunity to serve as your Senate confirmed Commissioner of U.S. Customs and Border Protection over the past year,” Magnus said in his resignation letter. “It has been a privilege and honor to be part of your administration.”

The vacancy sets up a Senate confirmation clash for any Biden nominee to the post. Last year, Magnus faced questions from both parties about the agency’s past actions, the treatment of migrants at the border and the Biden administration’s approach to immigration enforcement.

Magnus’s resignation comes as the Biden administration struggles to manage increasing migration at the U.S.-Mexico border. Fiscal 2022 was the busiest year on record at the border, with agents recording nearly 2.4 million encounters with migrants.

Some migrants are turned away under pandemic-era border restrictions under Title 42, but many others are allowed into the U.S. to seek asylum, contributing to lengthy case backlogs and straining border communities.

More recently, state and local leaders from elsewhere in the U.S. have asked for federal assistance accommodating migrants, including Washington, D.C. Mayor Muriel Bowser and Massachusetts Gov. Charlie Baker. Republican governors have sent migrants to areas they perceive as politically liberal, including Chicago, New York City and Martha’s Vineyard, Mass., to voice opposition to Biden administration immigration policies.

Magnus, the former police chief of Tucson, Ariz., was first confirmed to his post in December 2021 — the first time the agency had a confirmed commissioner since 2019. At his October 2021 confirmation hearing, Magnus promised to balance humane treatment of asylum seekers and border security.

His agency has faced repeated criticism from immigrant advocates and Democrats over treatment of migrants at the border. Last September, CBP came under fire after the circulation of images showing border patrol agents on horseback confronting Haitian migrants with reins.

Magnus has also drawn criticism from conservatives who want the Biden administration to crack down on illegal border crossings and end policies they say have encouraged migrants to come to the border, including a partial rollback of Title 42 for unaccompanied children and some family units.

Earlier this month, a group of House Republican lawmakers led by Georgia Rep. Jody Hice urged Biden to fire Magnus, citing a recent Politico report where other CBP officials expressed frustration with his job performance.

According to that report, Magnus was “unengaged in his job” and missed meetings about the situation at the U.S.-Mexico border. The officials also said Magnus badmouthed other officials and “has not built relationships within CBP and across other agencies to address the influx of migrants at the border.”

Deputy CBP Commissioner Troy Miller will now serve as acting commissioner of the agency.