Of all the distortions and paranoia that Tucker Carlson promoted on his since-canceled Fox News program, one looms large: a conspiracy theory that an Arizona man working as a covert government agent incited the Jan. 6, 2021, assault on the Capitol to sabotage and discredit former President Donald J. Trump and his political movement.

What’s known about the man — a two-time Trump voter named Ray Epps — is that he took part in demonstrations in Washington that day and the night before. He was captured on camera urging a crowd to march with him and enter the Capitol. But at other points, he pleads for calm once it becomes clear the situation is turning violent. He can be seen moving past a line of Capitol Police at the barricades, but never actually goes inside the Capitol.

Federal prosecutors have not charged Mr. Epps with a crime, focusing instead on the more than 1,000 other demonstrators who acted violently or were trespassing in the Capitol. The Justice Department’s sprawling investigation into the attack remains open, however, and Mr. Epps could still be indicted.

Yet for more than 18 months, Mr. Carlson insisted that the lack of charges against Mr. Epps could mean only one thing: that he was being protected because he was a secret government agent. There was “no rational explanation,” Mr. Carlson told his audience, why this “mysterious figure” who “helped stage-manage the insurrection” had not been charged.

He repeated Mr. Epps’s name over and over — in nearly 20 episodes — imprinting it on the minds of his viewers.

Mr. Epps was in the Marine Corps but said in his deposition before the Jan. 6 committee that he had otherwise never worked on behalf of any government agency. He and his wife, Robyn, have fled Arizona and are in hiding in another state, having sold their wedding venue business and ranch after receiving death threats from people who appeared to believe the conspiracy theory. And his legal jeopardy is far from over given that prosecutors are still unsealing new cases in connection with Jan. 6.

Now lawyers representing Mr. Epps and his wife are proceeding with plans to sue Fox News for defamation. “We informed Fox in March that if they did not issue a formal on-air apology that we would pursue all available avenues to protect the Eppses’ rights,” said Michael Teter, a lawyer for Mr. Epps who sent the network a cease-and-desist letter asking for an on-air apology and a retraction. After Mr. Teter did not hear from Fox about his request, he began to prepare the suit. “That remains our intent.”

Mr. Epps declined to comment on his potential suit. A Fox News spokeswoman declined to comment.

Mr. Carlson also declined to comment. But he continues to push the false notion that the Jan. 6 attack was staged by anti-Trump elements inside the government. On a podcast last week, Mr. Carlson claimed that the riot “was not an insurrection” and that the crowd that day was “filled with federal agents.”

First Amendment experts say Mr. Epps has a viable case for defamation — one reminiscent of the lawsuit the network recently settled with Dominion Voting Systems for $787.5 million, a case centered on numerous examples of false statements made on Fox News programs over an extended period.

If Mr. Epps moves forward, the case would be another legal complication and reputational stain for the conservative network, which faces a growing list of lawsuits related to its airing of false claims about the 2020 election and its aftermath. They include a $2.7 billion suit from a second voting technology company, Smartmatic, and two separate claims by Fox Corporation shareholders. Another lawsuit from a former producer for Mr. Carlson, which Fox settled on June 30 for $12 million, alleged that he condoned and encouraged a toxic workplace.

A defamation suit by Mr. Epps would be further evidence of how Mr. Carlson continues to pose a headache for Fox well after the network relieved him of his anchoring duties. Fox executives took him off the air after his text messages, which became public as part of the Dominion suit, revealed he had expressed hateful and racist sentiments.

On the air, his behavior had begun to grate on senior Fox executives like Lachlan Murdoch, chief executive of Fox Corporation, who disliked Mr. Carlson’s continued promotion of conspiracy theories about Jan. 6, which had drawn rebukes from Republicans including Senator Mitch McConnell. On the day he was informed his show was canceled, Mr. Carlson had been planning to run another segment on Mr. Epps, according to a tweet from an authorized biographer of the host, Chadwick Moore.

By design, defamation law tilts heavily in the news media’s favor, making it difficult to be found liable for defaming public figures — who are often targets of media reporting — unless there is proof that the defendants either knew what they said was false or acted with a reckless disregard for the truth. Mr. Epps would be able to argue that Mr. Carlson repeatedly uttered statements about him from October 2021 to March 2023 that were baseless, or easily explained or contradicted by the facts reported in numerous news reports.

“His challenge is to get a judge, if he files the suit, to say this was so inherently, bizarrely improbable that only a reckless person would put it into circulation,” said Rodney Smolla, the president of Vermont Law School and a defamation expert who consulted for Dominion during its case against Fox News.

“No case is easy,” Mr. Smolla added, “but this one is certainly, in my view, viable.”

The attacks against Mr. Epps began circulating online after a video taken on the night before the Capitol attack. It shows Mr. Epps at a pro-Trump demonstration on a Washington street shouting that he planned to march to the Capitol and enter. After pausing for a few seconds, he adds, “Peacefully.” Some in the crowd begin chanting “Fed! Fed! Fed!” at him, implying he was a government agent trying to goad Trump supporters into committing a crime.

Another video, taken on Jan. 6, also shows Mr. Epps encouraging people to march toward the Capitol. Then he bends down to whisper in a man’s ear moments before the man and rioters overcome police officers and breach the security perimeter around the Capitol grounds. It is difficult to hear what Mr. Epps says in the video.

Law enforcement immediately took note of Mr. Epps’s suspicious behavior and put a picture of him on an online wanted list. Mr. Epps has said he called the F.B.I.’s National Threat Operations Center shortly after the alert went up, and his phone records show he spoke to agents there for nearly an hour.

When the bureau removed him from the list — a few months after agents formally interviewed him and his son in the spring of 2021 — Mr. Carlson and others claimed that Mr. Epps’s disappearance and the lack of criminal charges meant that the government was protecting him.

On his programs, Mr. Carlson claimed that Mr. Epps was a liar and demanded that he be arrested. In one segment that ran shortly before Fox News canceled Mr. Carlson’s show in April, he showed viewers an image of the FedEx logo that had been altered to say “FedEpps.”

The fact that Mr. Epps has not been charged is largely in keeping with hundreds, if not thousands, of individual decisions that the Justice Department has made in its vast investigation of the Capitol attack.

Only a handful of people who pushed past barriers at the Capitol but never went inside the building have faced charges, and no defendants have been charged with incitement. Incitement charges against Mr. Epps would be particularly hard to prove given that he ultimately sought to de-escalate the crowd, and his most vocal encouragement to enter the building took place on the night before the attack, making it almost impossible to show his words had an immediate effect.

What Mr. Epps whispered to that man on the day of the attack has been answered three separate times: in an interview the F.B.I. conducted with the man Mr. Epps had talked to, Ryan Samsel; in Mr. Epps’s own interview with the authorities; and in a podcast interview with a co-defendant in Mr. Samsel’s case. All three said Mr. Epps had urged Mr. Samsel to calm down.

“He came up to me and he said, ‘Dude’ — his entire words were ‘Relax, the cops are doing their job,’” Mr. Samsel said, according to a recording of his interview with the F.B.I.

Mr. Carlson, in his legal defense, could point to inconsistencies in Mr. Samsel’s account. He could also note that Mr. Epps sent a text to a family member, well after the riot ended, saying he helped “orchestrate” the movements of people toward the Capitol.

(In recent weeks, Mr. Samsel has abruptly changed his story. From jail, he has started calling reporters — mostly for right-wing media outlets — to say that he lied to the F.B.I. and that Mr. Epps told him to pull at the barricades. Mr. Samsel acknowledged to The New York Times, however, that he had not provided this new story under oath to prosecutors.)

There are also unresolved legal questions about whether Mr. Epps truly suffered reputational harm if the only people he presumably lost esteem with are those who think Jan. 6 was a righteous cause.

“The question I would raise if I were Tucker Carlson’s lawyer,” said David A. Logan, a former dean of Roger Williams School of Law, “is should Epps be able to claim defamation when the people who think less well of him are criminals?”

“Courts have struggled with this exact question,” he added, pointing to hypotheticals like a man who sues over false allegations that he is gay or an anti-abortion activist who claims she was wrongly accused of having an abortion.

Mr. Carlson could also rely on the ambiguous and indirect language he sometimes used in describing Mr. Epps. For instance, he said at various points that he couldn’t be sure if Mr. Epps was really a double agent, acknowledging, “We don’t know anything about him.”

An indictment of Mr. Epps could also complicate his defamation case, by making any claim of reputational damage more difficult. “The centerpiece of a libel case is an alleged harm to reputation, so it for sure can become trickier to prove that you experienced a damages-incurring loss if your reputation is already poor because of true information,” said RonNell Andersen Jones, a professor at the University of Utah S.J. Quinney College of Law. “But the questions are often complex.”

Only if a judge allows a case to proceed, Mr. Logan said, will his lawyers know how strong their position is.

“Unlike Dominion, without Epps filing suit and getting broad discovery, we can’t be sure that Tucker Carlson had any doubts about the veracity of the allegations,” Mr. Logan said. “Or that similar doubts went up the corporate chain.”

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