A jury found Oath Keepers founder Elmer Stewart Rhodes guilty of engaging in a seditious conspiracy.
Three other members of the far-right group were found not guilty of joining in that conspiracy.
Legal experts said Tuesday's verdict in the high-stakes case will give the Justice Department momentum.
A jury on Tuesday found Oath Keepers founder Elmer Stewart Rhodes guilty of engaging in a seditious conspiracy to prevent the peaceful transfer of power from then-President Donald Trump to now-President Joe Biden, handing the Justice Department a victory in a prosecution featuring the most significant charges connected to the January 6, 2021, attack on the Capitol.
Rhodes was convicted of seditious conspiracy alongside Kelly Meggs, another member of the Oath Keepers. But the jury found three other members of the far-right group - Kenneth Harrelson, Jessica Watkins, and Thomas Caldwell - not guilty of joining them in that conspiracy.
All five were convicted of separate charges, including obstruction of an official proceeding, a felony that has led to some of the stiffest sentences in January 6 prosecutions.
"It's a really significant verdict. They could have just gone with the obstruction charge. But it was important to demonstrate that this really was a violent attempt to overturn the results of the election and interfere with the peaceful transfer of power. The jury's verdict confirms that that is what it was," said Randall Eliason, a George Washington University law professor and former top public corruption prosecutor in Washington, DC.
"Now, going forward, there are people higher up than the Oath Keepers who were potentially involved in the conspiracy," he told Insider. "This verdict gives the Justice Department some momentum in going up the ladder."
The jury reached the verdict after three days of deliberations that were interrupted by the Thanksgiving holiday, capping a high-stakes trial that lasted nearly two months. The verdict came just weeks before members of another far-right group, the Proud Boys, are set to stand trial on seditious conspiracy and other charges related to the Capitol attack.
Responding to the verdict, Attorney General Merrick Garland said it reaffirmed the Justice Department's commitment "to holding accountable those criminally responsible for the assault."
During the weekslong trial, federal prosecutors presented evidence of the Oath Keepers planning ahead of the January 6 attack, and they showed jurors images of some members of the far-right group entering the Capitol in a military-style stack formation.
In several messages shown at trial, the Oath Keepers discussed January 6 as a moment for revolution - with one member saying the group "would be in the lead of 1776.2," an apparent reference to the year in which American colonies declared independence from Britain.
"They claimed to wrap themselves in the Constitution. They trampled it instead," said prosecutor Jeffrey Nestler, in a closing address to jurors. "They claimed to be saving the Republic, but they fractured it instead."
Earlier in the trial, prosecutors played audio of a November conference call - recorded by at least one Oath Keepers member - in which Rhodes said the group was "very much in exactly the same spot that the Founding Fathers were in like March 1775."
"There's going to be a fight. But let's just do it smart and let's do it while President Trump is still commander in chief and let's try to get him to do his duty and step up and do it," Rhodes said.
In one of the most dramatic moments of the trial, jurors heard from Rhodes in person.
Testifying in his own defense, Rhodes sought to distance himself from the conduct of fellow Oath Keepers members and denied that the far-right group planned ahead of January 6 to breach the Capitol building. He told jurors it was "stupid" for those fellow members to go inside the Capitol as lawmakers gathered to certify the results of the 2020 election.
At trial, federal prosecutors said the Oath Keepers seized on the opportunity to enter the Capitol as members of a pro-Trump mob breached and ransacked it on January 6. Prosecutors pointed to a cache of weapons the Oath Keepers kept at a hotel outside Washington, DC, for a so-called "quick reaction force" that the group could deploy into the nation's capital.
Just as he distanced himself from Oath Keepers who entered the Capitol, Rhodes sought to downplay the far-right group's references to quick reaction forces, or QRFs, in his testimony before jurors.
"It gets used too often, frankly, and becomes confusing," he said.
But with their relatively quick verdict, the jury rejected Rhodes' defense and efforts to separate himself from the events of January 6.
This is a developing story.