Former leader of far-right group the Proud Boys Enrique Tarrio has been convicted of orchestrating a plot to attack the US Capitol on 6 January 2021.
The attack led to members of the group storming the building after Joe Biden was officially confirmed as the next president.
Tarrio, along with three other members of the group, were found guilty of seditious conspiracy - which is conspiring against the legitimacy of the state.
During the election campaign, Donald Trump spoke directly to the Proud Boys, known as a militant, gun-carrying group, telling them to "stand back and stand by".
The charges carry up to 20 years in prison.
In a big milestone, it means the US Justice Department has now secured seditious conspiracy convictions against leaders of two right-wing groups, the other being the Oath Keepers, who were said to be doing everything they could to keep Joe Biden out of the White House.
Tarrio was not in Washington DC on the day of the attack - he had been arrested two days before and ordered to leave the city.
However, prosecutors said he led and orchestrated proceedings carried out by his group, which saw itself as "Trump's Army", on 6 January.
The Proud Boys were "lined up behind Donald Trump and willing to commit violence on his behalf", prosecutor Conor Mulroe said in his closing arguments during the trial.
Prosecutors used hundreds of messages between members of the group in evidence that showed the Proud Boys peddling conspiracy theories and misinformation about the notion of a "stolen election", and what might happen when Joe Biden took office.
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In one social message, Tarrio wrote while the attack was ongoing: "Do what must be done."
He later said in a group chat for his members to "do it again" and "make no mistake, we did this".
The defence said there was no plot to attack the Capitol, or prevent Congress from certifying Joe Biden's win, with an attorney for Tarrio even trying to blame Trump, saying he himself incited the riots.
"It was Donald Trump's words. It was his motivation. It was his anger that caused what occurred on January 6th in your beautiful and amazing city," attorney Nayib Hassan said in his final appeal to jurors.
"It was not Enrique Tarrio. They want to use Enrique Tarrio as a scapegoat for Donald J Trump and those in power."
Ethan Nordean, Joseph Biggs, Zachary Rehl and Dominic Pezzola were the other members also convicted, some of which led chapters of the Proud Boys in other states.
In recent weeks, it was reported former vice president Mike Pence gave testimony to the committee set up to investigate the riots, apparently giving seven hours of evidence.
Mr Pence was tasked with certifying the 2020 election result in Congress, and was reportedly pressed by Trump to ignore the result.
There were images and videos during the riots of people calling for the death of Mr Pence after he confirmed the election result.