The Defense Department is reviewing a request from the Department of Homeland Security for 20,000 National Guardsmen to help with the Trump administration’s mass deportation efforts, the chief of National Guard Bureau told lawmakers Tuesday.

At a House hearing Tuesday, Gen. Steven Nordhaus was asked about a report last week in The New York Times that Homeland Security officials requested the Guardsmen to help with “interior immigration enforcement” — which would mark a major change from the supporting role that the Guard has traditionally played in security at the southern border.

“I do know that the Department of Defense has received a request for assistance,” Nordhaus said. “It is just initially into the building and is being staffed. I don’t have specific details or how the department will execute on that request for assistance at this time.”

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The National Guard has buttressed U.S. Customs and Border Patrol agents at the U.S.-Mexico border for years, assisting with tasks such as patrols and building barriers.

But President Donald Trump, after winning with a presidential campaign message that vowed “mass deportations now,” has increasingly turned to the military to deliver that promise.

Trump has deployed roughly 10,000 U.S. troops to the border, as well as more heavy-duty equipment primarily meant for overseas combat, and given active-duty troops new roles such as patrols. He also designated a large stretch of land along the border as a military zone as a way to increase criminal charges against those arrested for crossing the border.

The military has also been tapped for deportation flights and housing migrants at the Guantanamo Bay detention center in Cuba while they await deportation.

If the National Guard is tasked with helping immigration enforcement inside the U.S., it would be an unprecedented mission.

At Tuesday’s hearing before the House Appropriations Committee’s defense subcommittee, Rep. Betty McCollum, D-Minn., pressed Nordhaus on several outstanding questions about the role the National Guard could play in deportations, including whether troops would be tasked with arrests, what legal authority they would have to conduct law enforcement activities, and what law enforcement training they would have.

“I think there’s a lot of legal questions that our Guardsmen and women need to know before they’re assigned to duties, and I expect due diligence to be done,” said McCollum, the ranking member of the subcommittee.

While Nordhaus had few specifics to offer, in general, he vowed that the National Guard would adhere to the Constitution.

“We operate under the authorities of either the state and the governor, or operate under authorities from Congress,” he said. “And so we take those things very seriously, and we make sure that we’re following the Constitution, we’re following the law, and any policy and guidance.”

He also appeared to downplay the unprecedented nature of the request, comparing it to the support the National Guard provides law enforcement at presidential inaugurations.

McCollum also pressed Nordhaus over how the Guard’s increasing mission on the border is affecting its ability to train for war.

Nordhaus brushed aside those concerns, saying that the Guard is “always ready, always there.” That stands in contrast to Nordhaus’ predecessor, who issued a stark warning last year about border missions distracting from training.

Related: How Trump Is Leaning on the Military to Fulfill His Hopes of Mass Deportations and an Immigration Freeze

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