National Guard Chief General Daniel Hokanson said in a memo that the National Guard lacked the number of volunteers requested for their mission on Capitol Hill following the breach of Congress on Jan. 6.
The memo circulated through the White House National Security Council last week and was obtained by Gillian Turner of Fox News.
Hokanson said that out of the 2,280 National Guard troops currently serving at the Capitol, only 500 have volunteered to extend.
"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," Hokanson said in the memo.
Defense Secretary Lloyd Austin overruled Hokanson on Tuesday, reported Fox News.
The memo comes after the Pentagon approved a request Tuesday from the Capitol Police to keep the National Guard in Washington D.C. through May.
The Pentagon said on Thursday that it will be requesting that governors volunteer full National Guard units rather than individuals.
According to another Fox News report, the Defense Department may be considering issuing involuntary activation orders to keep the National Guard stationed at the Capitol.
Involuntary activation orders can see National Guardsmen serve up to 30 days in any four-month period or up to 60 days in any two-month period. It is unclear at this time if the Department of Defense will issue this command.