Defense officials say they are worried about an inside attack or some other threat from service members involved in securing President-elect Joe Biden's inauguration, The Associated Press reported.
As a result, the FBI is vetting all service members on hand in the capital to support the inauguration, an Army official told NBC News on Sunday.
"The Army is working with the FBI to vet all service members supporting the Inauguration National Special Security Event," the official said.
Army Secretary Ryan McCarthy told the AP that officials were conscious of the potential threat, and he warned commanders to be on the lookout for any issues as the inauguration approaches.
Former and current members of law enforcement agencies and the military appear to have participated in the Capitol incursion. A corporal in the Virginia National Guard was charged last week in federal court in connection with the violence, which has been linked to five deaths and widespread damage throughout the building.
Defense officials have said they have seen no evidence of any threats.
"While we have no intelligence indicating an insider threat, we are leaving no stone unturned in securing the capital," acting Defense Secretary Chris Miller said in a statement Monday. "This type of vetting often takes place by law enforcement for significant security events."
FBI vetting would involve running peoples' names through databases and watchlists maintained by the bureau to see whether anything alarming comes up.
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The Army and the Justice Department are investigating members for involvement, as well, and U.S. Capitol Police announced last week that they had suspended "several" of their own and will investigate at least 10 officers.
The FBI's vetting of all service members is a change from last week, when the Army said it was working with the Secret Service to determine which troops would require additional background screening.
Security in Washington has been dramatically tightened since Jan. 6, when pro-Trump supporters overran the Capitol and tried to stop the certification of Biden as the next president. There are 25,000 National Guard troops in Washington for the inauguration, with more armed guard troops than in the past, a defense official said.
National Guard troops from around the country have filled Washington and the Capitol, with troops seen working there and resting between shifts on the marble floors of the building and under the busts of the nation's Founding Fathers last week. There are fears of armed protests and threats before and through the inauguration as right-wing extremists call for violence against government officials on the encrypted communication app Telegram.
The beefed-up security in and around the Capitol includes razor wire, military patrols and police roadblocks.
Two people have been arrested at security checkpoints in Washington: a Virginia man with an "unauthorized" inauguration pass, a gun and more than 500 rounds of ammunition and a woman pretending to be "part of the presidential Cabinet," NBC Washington reported.
Although security is tighter ahead of the inauguration, the presence of the National Guard in the capital ahead of the presidential transfer of power is "nothing out of the ordinary," and the guard has "been involved in inaugurations since George Washington," Lt. Col. Timothy Shubert, who has been commanding the National Guard members near the security checkpoint outside Union Station, said Sunday.
"It's a deterrence for anyone who wants to do bad things," Shubert said. "There's always a chance at something getting stirred up. This is to keep people safe, and understand that we're here for the American people. That's all it is."
It's not only in the heart of Washington that security preparations are underway. Mayor Muriel Bowser said the city is preparing for protests and violence away from the National Mall, as well.
"What you're showing is really the federal enclave of Washington, D.C., not where the 700,000 of us live," Bowser said on NBC News' "Meet the Press." "So our police department, working with our federal law enforcement partners and the United States Army, quite frankly, also has a plan to pivot if we have any attacks in our neighborhoods."