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File picture: US President-elect Joe Biden. Image Credit: AP

Washington: 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. Such groups "pose the most likely threat" to the inauguration, according to a joint threat assessment from the FBI and Department of Homeland Security, which warned that attackers could target federal buildings and public officials in the days leading to the inauguration of Joe Biden as the 46th president.

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 Pekoske, the TSA administrator, said in a statement Friday that the agency was vetting "hundreds of names" before the event on January 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. Two of the prominent airports close to Washington are actually in Virginia, which has more relaxed firearms laws.

"Our intelligence and vetting professionals are working diligently around the clock to ensure those who may pose a threat to our aviation sector undergo enhanced screening or are prevented from boarding an aircraft," Pekoske said, adding that several airlines had announced in recent days that they would not allow passengers to check in guns.

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. It was unclear how many suspects had been restricted from flying. Multiple Democrats in Congress demanded the move after the rampage at the Capitol.

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 Biden's decisive victory in the popular vote and Electoral College.

Since the January 6 attack on the Capitol, intelligence officials have seen efforts to fan the violent messaging, according to a joint threat assessment dated Thursday. The escalation is consistent with previous attempts to take advantage of divisive Republican talk, such as Russia's drive to amplify disinformation spread by President Donald Trump during the campaign about the security of mail-in voting.

Officials wrote in an intelligence bulletin obtained this week by The Times that extremists aiming to incite a race war "may exploit the aftermath of the Capitol breach by conducting attacks to destabilize and force a climactic conflict in the United States."

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.

But law enforcement remains concerned about potential threats throughout the country. There have been calls for armed protests in all 50 states, but it remains unclear how many will materialize and whether they pose any credible threats of violence.

Gov. Gretchen Whitmer has activated the Michigan National Guard to help with security in Lansing, where armed people flooded into the state Capitol last year to protest coronavirus restrictions and where 13 men were arrested in October on terrorism, conspiracy and weapons charges. At least six of them, officials said, had hatched a detailed plan to kidnap Whitmer, a Democrat who became a focal point of anti-government views and anger over coronavirus control measures.

Gov. Gavin Newsom of California authorized the deployment of 1,000 National Guard troops Thursday and surrounded the state Capitol grounds in Sacramento with a 6-foot covered chain-link fence to "prepare for and respond to credible threats."

Defense Department and National Guard officials said on Friday that they were pressing governors of all 50 states for reservists to fill a growing demand for security.

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 U.S. troops in Iraq, Afghanistan, Somalia and Syria, could still grow.

Among the most prized Guard units are military police. All Army National Guard troops are trained to deal with civil disturbances, but the military police Guard troops have additional training and expertise.

As Guard troops armed with M9 handguns and automatic rifles took up positions around the Capitol this week, lawmakers who had praised the decision by the Army secretary, Ryan McCarthy, to arm some troops expressed unease.

"I would always rather see the Guard in a supporting role for domestic missions," said Rep. Michael Waltz, R-Fla., a former Army Green Beret who is now a member of the Maryland National Guard. Having soldiers in law enforcement, he said, "makes me nervous."

The authorities are hoping to keep the public away from downtown Washington during the inauguration. The Office of Personnel Management advised federal agencies to find ways to allow employees to stay home next week. Mayor Muriel Bowser recommended the public tune into the event online.

The National Mall - an iconic arena of American celebration, protest and unity - will be closed until at least Thursday, the day after Biden's inauguration, the National Park Service announced Friday.

Two small areas adjacent to the 2-mile park, which extends from the foot of the Capitol to the Tidal Basin behind the Lincoln Memorial, will remain open for inauguration events, and areas will be set aside for peaceful protest, the service said in a statement.

"We saw white extremists storm the Capitol building who were trained and organized," Bowser said, adding, "We all have to think about a new posture."