Updated

A left-wing activist who told Fox News last week that he’d followed a pro-Trump mob into the Capitol in order to "document" the siege is now the subject of a criminal complaint in connection with his alleged participation, according to the U.S. Justice Department.

John Sullivan can allegedly be heard egging on protesters in video he provided to the FBI, according to a federal criminal complaint. He has also shared the video to his YouTube and Twitter accounts under the pseudonym Jayden X.

BLM ACTIVIST INSIDE CAPITOL CLAIMS HE WAS 'DOCUMENTING' RIOTS, ONCE SAID 'BURN IT ALL DOWN'

He was charged Thursday in federal court in Washington after being arrested by the FBI. He remains in custody in Toeele County, in his home state of Utah, on a U.S. Marshals Service hold request.

James Sullivan, the man’s brother, told Fox 13 News that he gave tips about his brother to the Salt Lake City FBI. James Sullivan said he found out that his brother was at the Capitol when friends posted pictures online.

The FBI’s Salt Lake City office did not immediately respond to an email from Fox News seeking comment.

(Toeele County Sheriff's Office)

BLM ACTIVIST INSIDE CAPITOL CLAIMS HE WAS 'DOCUMENTING' RIOTS, ONCE SAID 'BURN IT ALL DOWN'

Inside the building, he told rioters that "We got to get this s--- burned," and "it’s our house m------------," according to an affidavit signed by FBI Special Agent Matthew Foulger.

He faces federal charges of civil disorder, entering a restricted building and violent entry or disorderly conduct.

Sullivan also allegedly told Foulger that he had been in the Capitol during the riot, entering through a broken window while wearing a ballistic vest.

As protesters climbed over a wall near the Capitol entrance, he allegedly exclaimed in the video, "You guys are f------ savage. Let’s go!"

"There are so many people," Sullivan’s voice can be heard saying as the camera shows a large group of people making its way toward the building. "Let’s go. This s--- is ours! F-- yeah. I can’t believe this is reality."

"We accomplished this s---," he said at another point "We did this together. F--- yeah! We are all a part of this history."

Inside the building, Sullivan told rioters that "We gotta get this s--- burned," and "it’s our house m------------," according to <a href="https://www.justice.gov/opa/page/file/1354781/download" target="_blank">an affidavit</a> signed by FBI Special Agent Matthew Foulger.

Inside the building, Sullivan told rioters that "We gotta get this s--- burned," and "it’s our house m------------," according to an affidavit signed by FBI Special Agent Matthew Foulger. (FBI)

In an interview with Rolling Stone published Wednesday, Sullivan said he was putting on an act to build rapport with the protesters.

"I had to relate to these people, and build trust in the short amount of time I had there to get where I need to go," he told the magazine. "To the front of the crowd to see the dynamic between the police and the protesters, because nobody wants to see the backs of people’s heads from a far-off distance."

He apparently made it there. During the unrest, he tweeted up-close video of the moment a Capitol Police officer fatally shot Air Force veteran Ashli Babbitt during the riot. It had been taken just a few feet away, at the height of the chaos outside the Speaker’s Lobby.

A screengrab from Sullivan's video outside the Capitol

A screengrab from Sullivan's video outside the Capitol (FBI)

Investigators used this detail and other aspects of the video to confirm Sullivan’s alleged whereabouts within the Capitol, they said.

Sullivan, the founder of a Utah-based group called Insurgence USA, told Fox News last Thursday that he’d entered the besieged Capitol to "document" what was going on and to dispute "this narrative going around" that Antifa instigators had played a role in the riot.

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While he has denied ties to Antifa, or anti-fascists, in recent interviews, his group Insurgence USA had advertised an event called "Kick These Fascists out of DC" on Wednesday around the same time as a pro-Trump rally near the National Mall that preceded the Capitol chaos.

Another screengrab showing protective equipment

Another screengrab showing protective equipment (FBI)

After the Trump rally, a mob of demonstrators breached the Capitol, ransacked congressional offices, and stormed the Senate floor, prompting a chorus of condemnations from members of both major parties. The breach interrupted a joint session of Congress aimed at validating Joe Biden as president-elect. Congress reconvened later in the evening and finished the process.

Sullivan, who was still in custody Thursday, could not be reached for comment.

Fox News’  Edmund DeMarche and Sam Dorman contributed to this report.