After spending months downplaying the seriousness of the attack on the U.S. Capitol, Tucker Carlson is now spreading a conspiracy theory that the FBI helped to orchestrate the entire thing.

On Tuesday night, the Fox News host said there were still "basic factual matters that we don't understand" about the riots of January 6, including why a number of people known to have been present have yet to be charged.

Citing an article from the right-wing site Revolver News, Carlson claimed that this could be because the government is "hiding the identity of many law enforcement officers who were present at the Capitol on January 6," not just the officer who shot and killed Ashli Babbitt.

"According to the government's own court filings, those law enforcement officers participated in the riots, sometimes in violent ways," Carlson said.

"We know that because, without fail, the government has thrown the book at most of the people who were present in the Capitol on January 6. There was a nationwide dragnet to find them. Many of them are still in solitary confinement tonight. But strangely, some of the key people who participated on January 6 have not been charged.

"Look at the documents, the government calls those people 'unindicted co-conspirators.' What does that mean? Well, it means that in potentially every single case they were FBI operatives. Really? In the Capitol on January 6?"

Carlson went on to discuss documents relating to Thomas Caldwell, one of several members of the far-right Oath Keepers group charged in connection with the events of January 6.

Carlson pointed out that there were two unindicted co-conspirators mentioned in an affidavit against Caldwell, including one referred to as "Person Two." This person stayed in the same hotel room as him and is alleged to have "stormed the barricades" with him on January 6.

Caldwell also said he believed there would be a "quick reaction force" operating on January 6, led by someone called "Person Three" in the document.

"But wait, here's the interesting thing. Person Two and Person Three were organizers of the riot," Carlson added. "The government knows who they are, but the government has not charged them. Why is that? You know why? They were almost certainly working for the FBI."

Carlson again cites the Revolver News report claiming that there could be as many as 20 unindicted co-conspirators in the Oath Keeper indictments who have not been charged, despite being accused of "virtually the exact same activities, and in some cases, much, much more severe activity" as the people named alongside them.

"So it turns out that this white supremacist insurrection was, again by the government's own admission in these documents, organized at least in part by government agents," Carlson said.

Carlson then plays a clip of FBI Director Christopher A. Wray admitting that the government has aimed to infiltrate extremist groups such as the Oath Keepers in order to get "better sources, better information, better analysis" to prevent an attack like January 6 from happening again.

Analyzing Tucker Carlson's claims about January 6

The main crutch of Carlson's conspiracy theory appears to be pointing out two known facts and suggesting that they are somehow linked: first, that the Capitol Police have not released the identities of some officers involved on January 6 and, second, that there are unnamed people mentioned in some suspects' affidavits.

There is no real reason to conclude that unindicted co-conspirators are government agents, with legal experts even noting that an undercover agent cannot be listed as a co-conspirator in court filings.

"There are many reasons why an indictment would reference unindicted co-conspirators, but their status as FBI agents is not one of them," Jens David Ohlin, a criminal law professor at Cornell Law School, told The Washington Post.

Lisa Kern Griffin, a professor at Duke University Law School, added: "Undercover officers and informants can't be 'co-conspirators' for the purposes of establishing an agreement to violate the law, because they are only pretending to agree to do so.

"An unindicted co-conspirator has committed the crime of conspiracy, and investigative agents doing their jobs undercover are not committing crimes."

The Post article noted that there were other reasons why a person might be referred to as an unindicted co-conspirator. These include that prosecutors don't know their name or don't have enough evidence to charge them yet, so would run the risk of compromising the investigation if they identified them.

In a 2004 paper, Ira Robbins, a law professor at American University in Washington, D.C., explores other reasons why a person may be referred to as an unindicted co-conspirator in criminal cases, including that the person is co-operating.

Speaking to fact-checking website Snopes, which has rated Carlson's FBI claim as false, Robbins described the conspiracy that the FBI organized the attack on the Capitol as a "speculative inflammatory leap that may play well with their audiences, but which may lack any basis in reality."

The two unnamed people in Caldwell's indictment can also be identified from other documents.

"Person Two" is Caldwell's wife, Sharon. She is named in another affidavit against Caldwell and fellow Oath Keepers Donovan Crowl and Jessica Watkins. In the document, it is noted that Thomas and Sharon Caldwell stayed together at the Comfort Inn Ballston.

In the same document, "Person Three" is identified by his first name as Paul, who stayed at the same hotel with another person.

In a motion arguing that Caldwell should not be held in custody before trial, his attorneys said the "quick reaction force" described by Caldwell as being headed by Person Three was not quick, reactive or a force—and was in fact just one person.

"Person Three—the Quick Reaction Force—is in his late 60s, obese, and has cardiopulmonary issues, a bad back, a bum knee, and is need of a hip replacement," the motion states.

The source of the article that Carlson discussed on air should also raise questions.

Although the Fox host described Revolver News as "one of the last honest outlets on the internet" on Tuesday, the website is run by Darren Beattie, a former Donald Trump speechwriter who was fired after it emerged that he had appeared at a conference frequently attended by white nationalists.

Newsweek has contacted Fox News for comment.

Tucker Carlson discusses populism during the National Review Institute's Ideas Summit in Washington, D.C. on March 29, 2019. The Fox News host has spent months downplaying the seriousness of the Capitol riots.