A New York man pleaded guilty Wednesday to storming the U.S. Capitol with fellow members of the far-right Proud Boys, a milestone in the Justice Department's prosecution of extremists who joined the Jan. 6 insurrection.

Matthew Greene is the first Proud Boys member to publicly plead guilty to conspiring with other members to stop Congress from certifying the Electoral College vote. He will also cooperate with authorities under the terms of his plea agreement.

Greene was arrested in April after a grand jury indicted him in the same case as two other alleged Proud Boys, Dominic Pezzola and William Pepe. They have pleaded not guilty.

Greene traveled from Syracuse, New York, to Washington, D.C., with Pezzola and other Proud Boys on Jan. 5. Prosecutors allege Greene advanced past toppled police barricades and was at the front of a mob when police began using pepper spray and other crowd-control measures.. But prosecutors have said they don't have any evidence that Greene entered the Capitol building that day.

"After the riot, (Greene) engaged in conversations (on) encrypted messaging platforms admitting to his role in the riot, encouraging others not to give up in a fight to take back their country, and comparing the situation as it existed in the weeks following January 6 to a fourth-generation war," prosecutors wrote in a June court filing.

More than three dozen people charged in the Capitol siege have been identified by federal authorities as Proud Boys leaders, members or associates, including at least 16 defendants charged with conspiracy. In a key case, four group leaders were charged in March with conspiring to impede Congress' certification of President Joe Biden's electoral victory.

Greene is expected to face a maximum of just over four years in prison at a sentencing set for March, and pay a fine of $15,000 to $150,000, along with about $2,000 in restitution. He pleaded guilty to federal charges of conspiracy and obstruction of an official proceeding.

Other extremist group members have been charged with conspiring to carry out a coordinated attack on the Capitol, including more than 20 people linked to the anti-government Oath Keepers.

Graydon Young, 55, of Englewood, Florida, was the first defendant to plead guilty in the Justice Department's major conspiracy case against Oath Keepers members. At least four others linked to the Oath Keepers also have pleaded guilty to riot-related charges.

Proud Boys members describe themselves as a politically incorrect men's club for "Western chauvinists." Its members frequently have engaged in street fights with antifascist activists at rallies and protests. Vice Media co-founder Gavin McInnes, who founded the Proud Boys in 2016, sued the Southern Poverty Law Center for labeling it as a hate group.

Police arrested the Proud Boys' top leader, Enrique Tarrio, in Washington two days before the Capitol riot and charged him with vandalizing a Black Lives Matter banner at a historic Black church during a protest in December 2020. Tarrio, who is serving his jail sentence for that case in the District of Columbia, hasn't been charged in connection with the Capitol siege.