EXCLUSIVE: Defense Secretary Lloyd Austin overruled the chief of the National Guard Bureau with the decision to keep 2,280 National Guard troops at the U.S. Capitol, according to a memo obtained by the Fox News Channel.

Gen. Daniel R. Hokanson, chief of the National Guard Bureau, laid out in the memo his assessment that the National Guard cannot and should not fulfill the Capitol Police's troop request at this time, making the case that the National Guard is already over-stretched due to coronavirus constraints, civil disturbances and wildfires.

"Efforts to date have not secured enough volunteers among supporting states to meet the USCP request of 2,280 soldiers, nor Option B of 1000 soldiers," the memo stated.

PENTAGON CONSIDERS INVOLUNTARY ACTIVATION OF NATIONAL GUARD TROOPS FOR US CAPITOL SECURITY

"I am concerned that the continued indefinite nature of this requirement may also impede our ability to man future missions as both adjutants general and guardsmen alike may be skeptical about committing to future endeavors," the memo stated.

U.S. Army Michigan National Guard Sgt. Hannah Boulden, from Ann Arbor, Mich., plays her guitar and sings in the Rotunda of the U.S. Capitol, before the vote on the Democrat's $1.9 trillion COVID-19 relief bill, on Capitol Hill, Wednesday, March 10, 2021, in Washington. (AP Photo/Alex Brandon)

The memo circulated within the White House National Security Council over the past week, Fox News Channel learned.

MICHIGAN NATIONAL GUARD MEMBERS COMPLAIN OF UNDERCOOKED, CONTAMINATED MEALS WHILE IN DC

"Yes, the Department of Defense will be funding this as we've funded the previous mission, which ends at the end of the week," Pentagon press secretary John Kirby said on Wednesday. "But that's not how anybody's looking at this or foisting that on the Capitol Police, that they're looking at this as free labor." 

"They have a legitimate need," he continued. "I'm not going to speak to specific threat. And as I've been trying to say over the last few days, it's not just about the threat environment in a highly polarized, hypercharged environment that we're in right now. It is very much about a capacity assistance to the Capitol Police as they begin to flesh out and develop what they're going to need long term to deal with a new reality on Capitol Hill."

The Department of Defense is reportedly considering issuing involuntary activation orders to keep National Guard troops stationed at the U.S. Capitol.

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The deliberation on a mandatory activation comes after Austin said he will be extending the National Guard's time stationed at the U.S. Capitol until May following a request from Capitol Police.

Fox News' Lucas Tomlinson and Caitlin McFall and the Associated Press contributed to this report.