General Austin added later, “We can do a better job of screening the folks we bring in.”
The F.B.I. investigation into the Capitol rampage, still in its very early stages, has identified at least six suspects with military links out of the more than 100 people who have been taken into federal custody or the larger number still under investigation. They include a retired Air Force lieutenant colonel from Texas, an Army officer from North Carolina and an Army reservist from New Jersey. Another person who served in the military was shot and killed in the assault.
The military’s examination of its ranks is a turning point for the Pentagon, which has a history of downplaying the rise of white nationalism and right-wing activism, even as Germany and other countries are finding a deep strain embedded in their armed forces.
Federal officials are vetting thousands of National Guard members arriving to help secure the inauguration. Of the 25,000 Guard personnel who are in Washington, any who will be near Mr. Biden or Ms. Harris will receive additional background checks, a standard procedure to counter insider threats that was also taken before Mr. Trump’s inauguration in 2017.
In addition to the National Guard members, the Pentagon plans to deploy about 2,750 active-duty personnel in support of the event. About 2,000 of them will perform ceremonial duties in military bands, color guards and a salute-gun battery, and serve as sentries and ushers.
The remaining 750 are members assigned to specialized units dealing with chemical, biological, nuclear or radiological threats; bomb squad technicians; medical personnel (including those conducting coronavirus testing in support of the attending physician of Congress); and logistics and communications support personnel.
Coast Guard helicopters and vessels will be in the air and nearby waterways.
Air Force fighter jets stationed at Joint Base Andrews in Maryland will be aloft over the region. The air space over the Capitol, the National Mall and the rest of the city will be even more restricted than usual, Pentagon officials said.
Jennifer Steinhauer contributed reporting.