WASHINGTON — The video’s title was posed as a question, but it left little doubt about where the men who filmed it stood. They called it “The Coming Civil War?” and in its opening seconds, Jim Arroyo, who leads an Arizona chapter of Oath Keepers, a right-wing militia, declared that the conflict had already begun.

To back up his claim, Mr. Arroyo cited Representative Paul Gosar of Arizona, one of the most far-right members of Congress. Mr. Gosar had paid a visit to the local Oath Keepers chapter a few years earlier, Mr. Arroyo recounted, and when asked if the United States was headed for a civil war, the congressman’s “response to the group was just flat out: ‘We’re in it. We just haven’t started shooting at each other yet.’”

Less than two months after the video was posted, members of the Oath Keepers were among those with links to extremist groups from around the country who took part in the Jan. 6 attack on the Capitol, prompting new scrutiny of the links between members of Congress and an array of organizations and movements that espouse far-right beliefs.

Nearly 150 House Republicans supported President Donald J. Trump’s baseless claims that the election had been stolen from him. But Mr. Gosar and a handful of other Republican members of the House had deeper ties to extremist groups who pushed violent ideas and conspiracy theories and whose members were prominent among those who stormed the halls of Congress in an effort to stop certification of President-elect Joseph R. Biden Jr.’s victory.

Their ranks include Representative Andy Biggs of Arizona, who like Mr. Gosar was linked to the “Stop the Steal” campaign backing Mr. Trump’s effort to overturn the election’s outcome.

Representative Lauren Boebert of Colorado has close connections to militia groups including the so-called Three Percenters, an extremist offshoot of the gun rights movement that had at least one member who entered the Capitol on Jan. 6.

Representative Marjorie Taylor Greene has promoted the QAnon conspiracy theory, whose adherents were among the most visible of those who stormed the building, and she appeared at a rally with militia groups.

Before being elected to Congress last year, Ms. Greene used social media in 2019 to endorse executing top Democrats and has suggested that the 2018 shooting at Marjory Stoneman Douglas High School in Parkland, Fla., was a staged “false flag” attack. The liberal group Media Matters for America reported on Thursday that Ms. Greene also speculated on Facebook in 2018 that California wildfires might have been started by lasers from space, promoting a theory pushed by followers of QAnon.

Representative Matt Gaetz of Florida appeared last year at an event also attended by members of the Proud Boys, another extremist organization whose role in the Jan. 6 assault, like those of the Oath Keepers and Three Percenters, is being investigated by the F.B.I.

It is not clear whether any elected officials played a role in directly facilitating the attack on the Capitol, other than helping to incite violence through false statements about the election being stolen from Mr. Trump. Officials have said they are investigating reports from Democrats that a number of House Republicans provided tours of the Capitol and other information to people who might have gone on to be part of the mob on Jan. 6. So far, no evidence has surfaced publicly to back up those claims.

Ms. Boebert said in a statement that she had “never given a tour of the U.S. Capitol to anyone besides family members in town for my swearing-in,” and she called accusations from Democrats that she gave a “reconnaissance tour” to insurgents an “irresponsible lie.” After the riot at the Capitol, she said she did not support “unlawful acts of violence.”

Mr. Biggs has denied associating with Stop the Steal organizers and condemned violence “of any kind.”

“Were you aware of any planned demonstration or riot at the U.S. Capitol to take place after the rally on Jan. 6, 2021? No,” Mr. Biggs said in a statement.

A spokesman for Ms. Greene said she now rejects QAnon, and he tried to distance her from militia members.

“She doesn’t have anything to do with it,” her communications director, Nick Dyer, said of QAnon. “She thinks it’s disinformation.” As for the militia members, he said, “Those people were at one event independently of Congresswoman Greene.”

Mr. Gosar did not respond to requests for comment.

Mr. Gaetz, on his podcast, said the Proud Boys were at the event he attended to provide security, and that “just because you take a picture with someone,” it does not mean “you’re tied to every viewpoint they’ve ever had or that they will ever have in the future.”

But in signaling either overt or tacit support, a small but vocal band of Republicans now serving in the House provided legitimacy and publicity to extremist groups and movements as they built toward their role in supporting Mr. Trump’s efforts to subvert the outcome of the 2020 election and the attack on Congress.

Aitan D. Goelman, a former federal prosecutor who helped convict the Oklahoma City bomber Timothy McVeigh, said that when elected officials — or even candidates for office — took actions like appearing with militia groups or other right-wing groups it “provides them with an added imprimatur of legitimacy.”

An examination of many of the most prominent elected Republicans with links to right-wing groups also shows how various strands of extremism came together at the Capitol on Jan. 6.

• In July, Mr. Gosar, a dentist, posed for a picture with a member of the Proud Boys. Two years earlier, he spoke at a rally for a jailed leader of Britain’s anti-immigrant fringe in London, where he vilified Muslim immigrants as a “scourge.” And in 2014, he traveled to Nevada to support the armed standoff between law enforcement and supporters of the cattle rancher Cliven Bundy, who had refused to stop trespassing on federal lands.

• Mr. Biggs, the chairman of the hard-right House Freedom Caucus, was seen by leaders of the Stop the Steal movement as an inspiration and has spoken at events hosted by extremists, including one at which a founder of the Oath Keepers called for hanging Senator John McCain.

• Ms. Boebert, elected to the House in November, said on Twitter that “Today is 1776” on the morning of Jan. 6, and she has connections to the Three Percenters, which shares her view that gun rights are under assault. At least one member of the group has been arrested in the breach of the Capitol.

• Ms. Greene has for years trafficked in conspiracy theories, expressed support for QAnon and made offensive remarks about Black people, Jews and Muslims. She also appeared at a campaign event alongside members of the Three Percenters.

To some degree, the members of Congress have been reflecting signals sent by Mr. Trump.

During a presidential debate in October, he made a nod toward the Proud Boys, telling them to “stand back and stand by.” Two months earlier, Mr. Trump described followers of QAnon — several of whom have been charged with murder, domestic terrorism, planned kidnapping and, most recently, storming the Capitol — as “people that love our country,” adding that “they do supposedly like me.”

Stop the Steal

Few Republicans have been more linked to extremist groups than Mr. Gosar.

“He’s been involved with anti-Muslim groups and hate groups,” said Mr. Gosar’s brother Dave Gosar, a lawyer in Wyoming. “He’s made anti-Semitic diatribes. He’s twisted up so tight with the Oath Keepers it’s not even funny.”

Dave Gosar and other Gosar siblings ran ads denouncing their brother as a dangerous extremist when he ran for Congress in 2018. Now they are calling on Congress to expel him.

“We warned everybody how dangerous he was,” Dave Gosar said.

In the days after the 2020 election, Mr. Gosar and Mr. Biggs helped turn Arizona into a crucible for the Stop the Steal movement, finding common cause with hard-liners who until then had toiled in obscurity, like Ali Alexander. The two congressmen recorded a video, “This Election Is A Joke,” which was viewed more than a million times and spread disinformation about widespread voter fraud.

Mr. Alexander has said he “schemed up” the Jan. 6 rally with Mr. Gosar, Mr. Biggs and another vocal proponent of Stop the Steal, Representative Mo Brooks of Alabama. Mr. Alexander’s characterization of the role of the members of Congress is exaggerated, Mr. Biggs said, but the lawmakers were part of a larger network of people who helped plan and promote the rally as part of Mr. Trump’s efforts to overturn the will of the voters.

After the election, Mr. Alexander emerged as a vocal proponent of the president’s stolen election claims, setting up a Stop the Steal website on Nov. 4 and making incendiary statements. On Dec. 8, he tweeted that he was willing to give up his life to keep Mr. Trump in office.

The Arizona Republican Party followed up, retweeting Mr. Alexander’s post and adding: “He is. Are you?” Mr. Alexander has since been barred from Twitter.

Ten days later, Mr. Gosar was one of the headliners at a rally in Phoenix that Mr. Alexander helped organize. Mr. Gosar used the rally to deliver a call to action, telling the crowd that they planned to “conquer the Hill” to return Mr. Trump to the presidency.

During his time onstage, Mr. Alexander called Mr. Gosar “my captain” and added, “One of the other heroes has been Congressman Andy Biggs.”

Although Mr. Biggs has played down his involvement with the Stop the Steal campaign, on Dec. 19, Mr. Alexander played a video message from Mr. Biggs to an angry crowd at an event where attendees shouted violent slogans against lawmakers. At the event, Mr. Biggs’s wife, Cindy Biggs, was seen hugging Mr. Alexander twice and speaking in his ear.

In 2019, Mr. Biggs spoke at an event supported by the Patriot Movement AZ, AZ Patriots and the American Guard — all identified as hate groups by the Southern Poverty Law Center, according to The Arizona Republic. In 2015, he sat silent at an event as a founder of the Oath Keepers called for the hanging ​of Senator McCain, calling him a traitor to the Constitution. Mr. Biggs told The Republic at the time that he did not feel it was his place to speak up and denounce the comments.

Mr. Arroyo, of the Oath Keepers in Arizona, said Mr. Gosar had attended two of their meetings, about a year apart. Mr. Arroyo said that his organization “does not advocate for breaking the law” and that he was “saddened to see the display of trespassing on the Capitol building by a few out-of-control individuals.”

Just like Mr. Gosar’s family, Mr. Biggs’s two brothers have publicly denounced him, saying he was at least partly responsible for the violence on Jan. 6. In addition, a Democratic state representative in Arizona, Athena Salman, has called on the Justice Department to investigate the actions of Mr. Gosar and Mr. Biggs before the riot, saying they “encouraged, facilitated, participated and possibly helped plan this anti-democratic insurrection.”

‘I Am the Militia’

In December 2019, hundreds of protesters descended on the Colorado Statehouse to oppose a new state law meant to take firearms out of the hands of emotionally disturbed people.

Among those at the rally were members of the Three Percenters, which federal prosecutors describe as a “radical militia group,” and a congressional hopeful with a history of arrests named Lauren Boebert, who was courting their votes. Armed with her own handgun, she posed for photographs with militia members and defiantly pledged to oppose the law.

In the months that followed, militia groups would emerge as one of Ms. Boebert’s crucial political allies. As her campaign got underway last year, she wrote on Twitter, “I am the militia.”

Militia members provided security for her campaign events and frequented the restaurant she owns, Shooters Grill in Rifle, Colo. In a recently posted video, a member of the Three Percenters was filmed giving Ms. Boebert a Glock 22 handgun.

Another member of the group, Robert Gieswein, who posed for a photograph in front of Ms. Boebert’s restaurant last year, is facing federal charges in the storming of the Capitol and attacking the police.

Photographs from the attack show him clad in tactical gear, goggles and a helmet, wrestling with Capitol Police officers to remove metal barricades and brandishing a baseball bat. Prosecutors have also cited a video of Mr. Gieswein encouraging other rioters as they smashed a window at the Capitol.

Once inside, Mr. Gieswein was photographed with another suspect, Dominic Pezzola, a former Marine and a member of the Proud Boys, who has also been charged in the Capitol attack.

Ms. Boebert’s communications director, Benjamin Stout, said in an email that she “has always condemned all forms of political violence and has repeatedly made clear that those who stormed the U.S. Capitol should be prosecuted to the full extent of the law.”

He added, “Simply because she takes a photo with someone that asks for one doesn’t mean she endorses every single belief they have or agrees with all other public statements or causes they support.”

The QAnon Caucus

One of the animating forces behind the attack on the Capitol was the movement known as QAnon, and QAnon has few more high-profile supporters than Ms. Greene.

QAnon is a movement centered on the fantastical claim that Mr. Trump, secretly aided by the military, was elected to smash a cabal of Democrats, international financiers and “Deep State” bureaucrats who worship Satan and abuse children. It prophesied an apocalyptic showdown, known as “the Storm,” between Mr. Trump and his enemies. During the Storm, their enemies, including Mr. Biden and many Democratic and Republican members of Congress, would be arrested and executed.

The mob that attacked the Capitol included many visible QAnon supporters wearing “Q” shirts and waving “Q” banners.

Among them was Jake Angeli, a QAnon devotee who styled himself the “Q Shaman.” Mr. Angeli, whose real name is Jacob Chansley, stormed the Capitol in horns and animal furs, and left a note threatening Vice President Mike Pence.

Also among them was Ashli Babbitt, a QAnon believer who was shot and killed by a Capitol Police officer as she tried to climb through a window in a barricaded door near the House chamber.

Ms. Greene was an early adherent, calling QAnon “a once-in-a-lifetime opportunity to take this global cabal of Satan-worshiping pedophiles out.” Many of her Facebook posts in recent years reflected language used by the movement, talking about hanging prominent Democrats or executing F.B.I. agents.

Ms. Greene has also displayed a fondness for some of the militia groups whose members were caught on video attacking the Capitol, including the Oath Keepers and the Three Percenters. Speaking in 2018 at the Mother of All Rallies, a pro-Trump gathering in Washington, she praised militias as groups that can protect people against “a tyrannical government.”