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.”