WASHINGTON (AP) — The head of U.S. Customs and Border Protection has resigned from his job leading the nation’s largest law enforcement agency as agents encounter record numbers of migrants entering the U.S. from Mexico.

Chris Magnus submitted his resignation to President Joe Biden on Saturday, saying it had been “a privilege and honor” to be part of the administration.

White House press secretary Karine Jean-Pierre said Biden had accepted Magnus' resignation. “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,” she said.

Two people who were briefed on the matter told The Associated Press on Friday that Magnus was told to resign or be fired less than a year after he was confirmed. The people spoke on condition of anonymity because they were not authorized to address the matter publicly.

Magnus’ removal is part of a larger shakeup expected at Homeland Security as it struggles to manage migrants coming from a wider range of countries, including Venezuela, Cuba and Nicaragua. This comes as Republicans are likely to take control of the House in January and are expected to launch investigations into the border.

Migrants were stopped 2.38 million times at the Mexican border in the fiscal year that ended Sept. 30, up 37% from the year before. The annual total surpassed 2 million for the first time in August and is more than twice the highest level during Donald Trump’s presidency, in 2019.

The Los Angeles Times was first to report on the ultimatum. In a statement to the newspaper, Magnus said he was asked by Homeland Security Secretary Alejandro Mayorkas to step down or be fired.