In the mosaic of moments that made up the attack on the Capitol on Jan. 6, it has long been one of the most intriguing: As a pro-Trump mob grew in size and anger, a man in a MAGA hat briefly spoke outside the building with the Proud Boys leader Joseph Biggs.

Minutes later, the man, Ryan Samsel, walked to the front of the crowd, turned his hat backward and started to shove aggressively at a barricade. As others joined him, the barricade fell, knocking down a police officer and arguably setting off the ensuing riot.

While the encounter between Mr. Biggs and Mr. Samsel was caught on video and is often thought of as a tipping point in the Capitol attack, little has been known about what the two men said to each other.

For months, however, according to three people familiar with the matter, the government has known Mr. Samsel’s account of the exchange: He has told investigators that Mr. Biggs encouraged him to push at the barricades and that when he hesitated, the Proud Boys leader flashed a gun, questioned his manhood and repeated his demand to move upfront and challenge the police.

Mr. Samsel’s version of events was provided to the government in late January, when he was interviewed by the F.B.I., without a lawyer present, shortly after his arrest in Pennsylvania, according to the people familiar with the matter. He has since been charged with several crimes, including assaulting an officer and obstructing Congress’ efforts to certify the election results.

It is not clear whether the F.B.I. views Mr. Samsel as credible at this point or whether the Justice Department intends to use his information in its case against Mr. Biggs.

A former airborne soldier who has helped organize several Proud Boys rallies in recent years, Mr. Biggs has been charged with conspiring with other leaders of the far-right group to march a mob of members toward — then into — the Capitol that day. He has not been charged with any gun crimes, and prosecutors never mentioned the accusation that he was carrying a weapon during the extensive arguments that preceded a judge denying him bail.

J. Daniel Hull, Mr. Biggs’s lawyer, denied that his client was armed on Jan. 6 or that he sought to encourage Mr. Samsel to confront the police, calling it a “desperate, if wildly entertaining, false history.”

“Not even the likes of the Biden Justice Department would buy into and spin a tall tale like that,” Mr. Hull said.

The New York Times reviewed two different versions of the video of the meeting, but the footage is brief and occasionally obscured by obstructions like a passer-by and a flag, making it impossible to conclusively confirm or deny Mr. Samsel’s account.

In its public filings, the government has never explicitly mentioned the video footage of the encounter between Mr. Biggs and Mr. Samsel, who appears to have no ties to the Proud Boys. Prosecutors have in fact left only meager hints about possible connections between the men, noting in court papers this spring that they were both in the same part of the crowd outside the Capitol.

But if Mr. Samsel’s account is true, it could serve to bolster arguments that some Proud Boys leaders intentionally incited ordinary people in the crowd — or what they refer to as “normies” — to commit violence during the attack. The government has offered other evidence, drawn from the group’s internal messaging chats, that two Proud Boys leaders from Philadelphia were excited by the prospect of “riling up the normies” on Jan. 6.

The vast investigation into the Capitol attack has so far led to 630 federal arrests and nearly 100 guilty pleas. The conspiracy charges against Mr. Biggs and 16 other Proud Boys are some of the most prominent allegations that the Justice Department has made.

Mr. Biggs, who lives in Florida, was the first of four Proud Boys leaders to be arrested in connection with the Capitol attack. The internal chats suggest that he was in communication with the group’s chairman, Enrique Tarrio, on the night before the assault.

Mr. Tarrio himself was not in Washington that day, having been ordered by a local judge to stay away from the city after his arrest days earlier on charges of illegally possessing ammunition magazines and burning a Black Lives Matter banner after a pro-Trump rally in December. He is currently serving a five-month sentence on the charges.

Before joining the Proud Boys, Mr. Biggs was, among other things, a correspondent for Infowars, the conspiracy-minded media company run by Alex Jones. He has remained close to Mr. Jones, who was also at the Capitol on Jan. 6 with his right-hand man, Owen Shroyer, who has been arrested on charges of illegally entering a restricted area outside the building. Mr. Jones has not been charged in connection with the riot even though he was standing near Mr. Shroyer.

The government has not yet secured Mr. Samsel’s cooperation in its investigation of Mr. Biggs, for reasons that remain unclear.

Mr. Samsel has been in custody since his arrest and has a history of violent behavior, prosecutors say, which could damage his credibility as a potential government witness. He has been convicted, for example, of crimes such as beating up his pregnant girlfriend and running a woman who owed him money off the road in his car.

Not long after he was jailed in connection with the Capitol riot, Mr. Samsel claimed a correctional officer assaulted him in a Washington jail, leaving him with a broken nose, a dislocated jaw and the loss of sight in his right eye. The fallout from the assault could also make it difficult for prosecutors to consider using his testimony.

His lawyer, Stanley Woodward, declined to comment on the case.