President Trump’s incitement of his supporters before their attack on the Capitol on Jan. 6 has galvanized a nationwide extremist movement and fueled those determined to disrupt the transfer of power to President-elect Joe Biden and violently challenge the legitimacy of the election for months — and possibly years, according to U.S. officials and independent experts.

U.S. officials have warned authorities nationwide to be on alert for potential acts of violence at state capitols, as well as a possible second attack on the Capitol or on the White House. Law enforcement authorities have said extremists might use firearms and explosives and are monitoring online calls to rally in cities nationwide beginning Sunday. Security at the inaugural ceremony in Washington on Wednesday probably will be the most intense ever.

At the center of the amorphous but increasingly motivated extremist movement sits the current president, now twice impeached, deprived of his social media megaphones but still exerting a powerful influence over his followers who take his baseless claims of election fraud as an article of faith.

Trump supporters who gathered to protest the certification of Joe Biden as the next U.S. President describe how they view the storming of the Capitol on Jan. 6. (The Washington Post)

It remains unclear when and where groups might launch follow-up attacks, but even if they do pull back in the days to come — and experts say there is some reason to think they might — the threat from Trump-inspired extremism is likely to remain and grow.

“It has begun to shift from ‘We are going to win this’ to ‘This fight is going to be a long one,’ ” said Rita Katz, executive director of the SITE Intelligence Group, which monitors extremist groups and their use of social media to inspire and organize adherents. “The prevalent consensus across the movements involved in or supporting the Capitol siege is that they will keep pushing forward.”

Federal authorities are warning state leaders to be prepared for the possibility of attacks in state capitals in the days before Biden’s inauguration.

New Mexico Gov. Michelle Lujan Grisham (D) declared a state of emergency on Thursday, saying it was “reasonable to believe” that rioters “will endanger the safety of legislators, legislative staff and the general public as well as destroy public and historic infrastructure” in the state.

“There are people in our country who want to turn peaceful protests into opportunities for violence,” Ohio Gov. Mike DeWine (R) said at a news conference announcing he would call up more than 400 National Guard troops and close state offices in Columbus until Jan. 21.

FBI Director Christopher A. Wray told reporters that officials were monitoring “an extensive amount of concerning online chatter” about events surrounding the inauguration.

“Right now, we’re tracking calls for potential armed protests and activity leading up to the inauguration,” Wray said, noting that it was a challenge “to distinguish what’s aspirational versus what’s intentional.”

Some officials said they aren’t taking any chances and were braced for the likelihood of attacks. After the Capitol siege, “what we’ve already seen and experienced is the depths and lengths people are willing to go in furtherance of their cause,” said Andrew Walsh, a deputy chief with the Las Vegas Metropolitan Police Department, who participated in a call with the FBI and local law enforcement officials on Wednesday.

“I’ll put it this way: People tell us what they’re going to do, and then they do it,” Walsh said. “After that, there’s no excuse to be surprised.”

As Trump spends his final days in office, some of his followers, bereft of his directions via Twitter, are unsure where to focus their energy, experts said. Some events planned in Washington and in state capitals have been canceled, partly out of fear that the events were actually organized by federal authorities as a “false flag” operation meant to trap Trump’s followers.

The organizer of the “Million Militia March” appears to have abandoned an Inauguration Day demonstration in Washington, warning followers on his personal website to “STAY FAR AWAY FROM DC & ALL STATE CAPITALS . . . IT IS A TRAP.”

Social media platforms and Web hosting services have removed accounts and chat apps that Trump supporters used to coordinate the Jan. 6 assault. But that also has made it harder for law enforcement officials to monitor those who might be planning violence.

It’s a measure of Trump’s influence that the extremists are expressing frustration that they are left without clear guidance by any central figure about what to do and when and where to gather. But they are adapting.

“What they’re now seeing is a fragmentation, a frustration among some of the followers because they don’t even have Trump giving instructions,” said one congressional aide, who spoke on the condition of anonymity to discuss sensitive government intelligence, including warnings contained in a joint intelligence bulletin issued by the FBI and the Department of Homeland Security. It concluded that there is “a great degree of uncertainty” around the timing and location of potential attacks, even as the volume of online extremist chatter has increased significantly.

The Anti-Defamation League said it has been tracking an uptick in calls for nationwide armed protests beginning Sunday.

Experts said far-right extremists have called on their followers to loot and burn buildings and engage in physical violence against those they perceive to have aided Biden in illegitimately seizing the presidency.

Several of the groups appear to have been deterred from a repeat protest in Washington, at least for now, by a massive show of military force. By next week, the D.C. police chief said, upward of 20,000 National Guard members are expected to be deployed in Washington.

On TheDonald.win, a forum that attracted some of Trump’s most zealous and threatening supporters in the run-up to the Jan. 6 attack, users appeared to accept that law enforcement officials probably would crush any protest in the near future. Participants on the forum discussed when the next opportunity for armed protest might arise.

“We won’t sit on our hands for the next four years but we can pick and choose our battles moving forward,” said one message written by Enrique Tarrio, the chairman of the far-right Proud Boys, and sent to subscribers of the group’s Telegram channel, an encrypted messaging app.

Thousands of users have flocked to Telegram as social media companies took steps to expel far-right groups and conspiracy theorists from their platforms. Twitter shut down more than 70,000 accounts affiliated with QAnon, a conspiracy theory that claims that a secretive cabal of Democratic Satan worshipers runs the government, and Amazon suspended Parler, a conservative alternative to Twitter, from its Web-hosting services.

Telegram has cracked down on its own sizable number of white-supremacist and neo-Nazi users, experts said. On Telegram, some users have called on followers to abandon plans for a second protest in Washington in favor of surprise attacks nationwide.

“These lizards have addresses outside of DC, all over the country, hell maybe nearby in your flyover state,” one channel administrator wrote. “Is the military going to deploy in mass everywhere? They can’t.”

Some members of Congress have sought out private personal security after Trump supporters harassed them at their homes, said one person familiar with the matter, who spoke on the condition of anonymity to protect the lawmakers from becoming bigger targets.

There are some indications that in the absence of clear direction from Trump, groups are taking it upon themselves to organize in their communities.

One Telegram message on a far-right channel called “Boogaloo Intel Drop” told followers to “get a feel for your local area and get your friends together.” The message said they should find others who are outraged about the death of Ashli Babbitt, who was shot by a police officer while storming the Capitol.

“No, we’re not going to tell you ‘show up on XX day and do XX,’ ” which would risk alerting authorities, the message continued, advising followers to “have some damn ingenuity and autonomy.”

In the week since the attack on the Capitol, members of the Boogaloo channel have been cheering the erosion of Trump supporters’ reverence for police and calling for continued actions. Boogaloos have long been advocates of violence against police and have not had the same qualms as other far-right groups about turning on law enforcement officers.

Experts said that although extremist groups are loosely organized, they are firmly united in their belief that the election was stolen, an idea that will persist long after Trump leaves the White House.

“This animating narrative that something has been taken away from them, stolen from them, is essentially a gift that has been given to them by the president, by others and will likely animate some extremists for the next four years at least,” said Oren Segal, the vice president of the Center on Extremism at the Anti-Defamation League.  

Katz, the terrorism researcher, said: “This movement isn’t something we are going to simply delete from social media or evaporate by keeping them off mainstream media. Just as we have seen with other extremist movements, the extremists behind the Capitol siege will become further radicalized as they are pushed into less-moderated online venues.”

Ellen Nakashima, Mark Berman and Marissa J. Lang contributed to this report.