Security tightens as law enforcement raises fresh alarms about extremist plots.

Law enforcement officials are vetting hundreds of potential airplane passengers and beefing up airport security as officials amplify warnings of violence before the presidential inauguration from extremists emboldened by the Capitol attack last week.

The Transportation Security Administration is increasing the number of federal marshals on flights and explosive-detection dogs at airports. Screening officers will be deployed to assist a militarized “green zone” in downtown Washington.

Federal officials say the security perimeter, which includes an increasing number of armed members of the National Guard, is necessary to prevent an attack from domestic extremists.

The extremists “remain a concern due to their ability to act with little to no warning, willingness to attack civilians and soft targets, and ability to inflict significant casualties with weapons that do not require specialized knowledge,” federal officials wrote in the bulletin obtained by The New York Times.

David P. Pekoske, the Transportation Security Administration administrator, said in a statement on Friday that the agency was vetting “hundreds of names” before the event on Jan. 20. Commercial airlines have tracked an increase in passengers checking in firearms on their way to airports in the Washington area, according to a separate bulletin from the Justice Department.

Federal agencies have also begun to identify those captured on video at the Capitol with weapons or engaging in violence and putting them on a “no-fly” list aimed at preventing suspected terrorists from boarding flights, according to an administration official.

Federal law enforcement officials have said they continue to be alarmed by an increase in chatter from groups like the boogaloo, a far-right group that aims to start a second civil war, and other racist extremists threatening to target the nation’s capital to protest Mr. Biden’s decisive victory in the popular vote and Electoral College.

Mr. Biden has resisted calls to move the celebration indoors for the sake of safety. His inauguration committee had already been planning a scaled-back celebration with virtual components because of the coronavirus.

National Guard officials said they would most likely need at least 25,000 troops in Washington, 5,000 more than they projected this week, for duties ranging from traffic control to security in and around the Capitol itself. That number, roughly more than three times the number of American troops in Iraq, Afghanistan, Somalia and Syria, could still grow.