Stewart Rhodes, leader of the Oath Keepers, was convicted of seditious conspiracy by a US jury on Tuesday for a plan to stop the peaceful transfer of power after Donald Trump lost the 2020 US presidential election to Joe Biden.
The sentence carries up to 20 years in prison.
Tuesday's verdict was a victory for the Justice Department, which has maintained that the January 6 attack on the Capitol was a violent "assault" on US democracy.
"Today the jury returned a verdict convicting all defendants of criminal conduct, including two Oath Keepers for seditious conspiracy against the United States," US Attorney General Merrick Garland said in a a statement.
"The Justice Department is committed to holding accountable those criminally responsible for the assault on our democracy on January 6, 2021."
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Donald Trump, the president at the time, speaks during a rally protesting the electoral college certification of Joe Biden as president in Washington on January 6, 2021. AP -
A mob loyal to Mr Trump tries to break through a police barrier at the Capitol. AP -
The mob waves pro-Trump flags in front of the Capitol building. AP -
Riot police push back a crowd of rioters at the Capitol building. AFP -
Trump supporters clash with police and security forces as people try to storm the US Capitol building. AFP -
Insurrectionists loyal to Mr Trump try to open a door of the US Capitol building as they riot in Washington. AP -
US Capitol Police push back rioters trying to enter the US Capitol. AP -
Police and rioters confront each other in the Rotunda of the Capitol. US Capitol Police via AP -
Smoke fills the hallway outside the Senate chamber of the Capitol. AP -
Insurrectionists loyal to Mr Trump breach the Capitol in Washington. AP -
Trump supporters, including Doug Jensen, centre, confront US Capitol Police in the hallway outside the Senate chamber at the Capitol. AP -
Newly installed razor wire tops the fence surrounding the US Capitol following the January 6 riot. Reuters -
Security agents and members of Congress barricade the door to the House chamber as the violent mob breaches the Capitol. AP -
Rioter Jacob Chansley holds a sign referencing QAnon as supporters of Mr Trump gather to protest the early results of the 2020 presidential election. Reuters
Rhodes founded the Oath Keepers, a right-wing militia group, in 2009.
Prosecutors said he stood outside the Capitol building on the day of the insurrection and acted as a general while the group's members stormed the building.
A leaked list in September showed that hundreds of elected officials, military personnel and law enforcement officers were on the militia group's membership list.
Kelly Meggs, who led the Oath Keepers inside the Capitol, was also found guilty of seditious conspiracy.
Co-defendants Kenneth Harrelson, Jessica Watkins and Thomas Caldwell were acquitted.
The Oath Keepers members were the first of about 800 people accused in the riot to go on trial.
The Justice Department said Rhodes and his co-defendants “concocted a plan for an armed rebellion … plotting to oppose by force the government of the United States”.
They were also accused of creating a “quick-reaction force” that prosecutors said was positioned at a nearby Virginia hotel and was equipped with weapons that could be sent to Washington if needed.
“As this case shows, breaking the law in an attempt to undermine the functioning of American democracy will not be tolerated,” FBI Director Christopher Wray said in a statement.
Fifty witnesses testified during the trial. Rhodes and two of his co-defendants testified in their own defence.
They denied planning an attack or trying to block Congress from certifying the 2020 election results.
The defendants said the trial was brought on by President Joe Biden's administration to punish supporters of Mr Trump.
Agencies contributed to this report
