Jeremy Bertino, 43, of Belmont, North Carolina, could potentially become a key witness against five other members of the group, including former Proud Boys Chairman Enrique Tarrio, who are due to stand trial beginning in December on charges including seditious conspiracy.

A sixth Proud Boys member, Charles Donohoe, 34, of Kernersville, North Carolina, pleaded guilty in April to conspiracy to obstruct an official proceeding and assaulting, resisting, or impeding officers.

Prosecutors said on Thursday that Bertino was not present in the Capitol on Jan. 6, 2021 because he was the victim of a stabbing during an altercation in December 2020 and was still recovering from his wounds.

However, they said he had previously accepted an invitation from Tarrio in December 2020 to join a new chapter called the "Ministry of Self Defense." Prosecutors said Bertino participated in chats with other members in an alleged plot to stop Congress from certifying Democrat Joe Biden's election win because they felt it had been stolen from Republican then-President Donald Trump.

As he recovered from his injuries, the government said he was also involved in private chats with other members who openly discussed the possibility of storming the Capitol, and he encouraged the group, telling them to "form a spear."

On his own social media account, he also posted messages such as "DO NOT GO HOME. WE ARE ON THE CUSP OF SAVING THE CONSTITUTION."

In addition to pleading guilty to seditious conspiracy, Bertino also pleaded guilty to a gun charge, after an FBI search of his home in March found he had six loaded firearms and more than 3,000 rounds of ammunition. He was barred from possessing guns due to a prior conviction of reckless endangerment in New York state in 2004, according to court filings.

Bertino's guilty pleas come as the first seditious conspiracy trial of Donald Trump supporters charged with organizing the Capitol assault is underway. That trial involves five associates of the far-right Oath Keepers, including founder Stewart Rhodes.

As part of his plea deal, Bertino must agree to testify before a grand jury or at trial if requested by federal prosecutors.

He could face a prison sentence of up to five years, three months and a fine of up to $200,000, officials said at a virtual hearing before U.S District Court Judge Timothy Kelly in Washington.

However, prosecutors said that if Bertino cooperates, they may seek a reduction in his sentence and he may be eligible for a witness security program.

Seditious conspiracy is a Civil War-era statute defined as two or more people plotting "to overthrow, put down or to destroy by force the government of the United States" that is rarely prosecuted.

In addition to Bertino, three members of the Oath Keepers - Joshua James, Brian Ulrich and William Todd Wilson - pleaded guilty this year to engaging in seditious conspiracy in connection with the attack.

They are awaiting sentencing and could potentially be called as witnesses against Rhodes and his co-defendants.

The storming of the Capitol by Trump supporters failed to stop the congressional certification of Biden's election victory and resulted in Trump's second impeachment. Five people died during and shortly after the riot, and about 140 police were injured.