The Biden administration’s rollback of Trump-era immigration policies has renewed focus on the mission at the U.S.-Mexico border, where thousands of troops are assisting federal agencies as the White House faces an uptick in migrant crossings.

President Biden on Thursday ended a 2019 national emergency that bolstered construction of a border barrier and sent additional soldiers to the area. The operation, which diverted billions from the Defense Department to help construct the wall, was “unwarranted,” Biden told lawmakers weeks after pausing construction of the wall as one of his first decrees.

Defense and White House officials have not said whether the mission, which is authorized through Sept. 30, will wind down early or be extended.

“I don’t have any changes to that mission to read out as a result of the president’s decision,” Pentagon spokesman John Kirby told reporters Thursday. The White House did not return a request for comment Friday.

Nearly 4,000 members of the National Guard are assigned to the border mission in support of law enforcement, carrying out such duties as aviation support, vehicle maintenance, and surveillance monitoring, the National Guard Bureau said. A contingent of about 1,900 National Guard members are detailed to separate drug interdiction operations.

During a deployment in 2018, soldiers also helped feed Border Patrol horses and in some cases shoveled their manure.

Guard members have been involved with the pandemic response, wildfire evacuations and civil unrest details over the past year, and nearly 25,000 were activated to support security operations for the presidential inauguration last month. Last year saw the most activations among National Guard members since World War II.

The Pentagon’s mission to support law enforcement along the border began in 2018, after Trump declared that migrant caravans traveling from Central America to the U.S. southern border amounted to “an invasion.”

Numbers of mobilized troops have fluctuated, though the highest mark came last summer with 5,500 troops deployed on the border, Pentagon spokesman Lt. Col. Chris Mitchell said.

“These missions can be supported with manageable impacts to readiness, and are contingent on the availability of funds and the continued statutory authority to provide such support,” Mitchell said.

The cost of the operation through last year was $939 million, according to Pentagon figures.

Uncertainty about the mission’s future comes as an influx of migrants could challenge the president’s plans to make the United States more welcoming to asylum seekers and refugees.

Arrests and detention cases rose to nearly 78,000 in January, the highest number for that month in at least a decade and more than double the amount from a year earlier. The numbers also rose in December despite a typical winter lull.

Nick Miroff contributed to this report.

Correction: The National Guard Bureau provided inaccurate data on the number of National Guard members supporting border operations. This story has been updated.