Accused Jan. 6 rioter who warned of possible 'civil war' expected to testify to House committee Tuesday

An Ohio man who accused Joe Biden, other Democrats, and the mainstream media of "treason" is set to testify in a public hearing Tuesday before the House committee investigating the attack on the U.S. Capitol last year, according to a source familiar with the matter.

The hearing is expected to focus on the rise of radical extremism in the United States, and the source said one of the key witnesses will be Stephen Ayres of Warren, Ohio, who recently admitted to illegally entering the Capitol on Jan. 6, 2021.

A former spokesman for the Oath Keepers militia group, Jason Van Tatenhove, will also be testifying Tuesday, the source said.

In court documents filed last month, Ayres acknowledged that the day before the riot, he drove to Washington, D.C., to protest Congress' certification of the 2020 presidential election results.

On Facebook, Ayres had spotlighted then-President Donald Trump's call for supporters to descend on Washington on Jan. 6, which Trump said will "be wild" in a Tweet he posted on Dec. 19, 2020.

During Tuesday's hearing, the committee hopes to explore the impact that Tweet had on Trump's supporters, committee aides said. The hearing is expected to also include clips from former White House counsel Pat Cipollone's deposition with congressional investigators last week, sources familiar with the matter told ABC News.

Two days before he left for the nation's capital, Ayres posted a message on Facebook saying, "Mainstream media, social media, Democrat party, FISA courts, Chief Justice John Roberts, Joe Biden, Nancy Pelosi, etc....all have committed TREASON against a sitting U.S. president!!! All are now put on notice by 'We The People!'"

In the week before that, Ayres said in social media posts that it was "time for us to start standing up to tyranny!" and he warned that "If the [deep state] robs president Trump!!! Civil War will ensue!" according to the FBI.

Ayres joined the mob outside the Capitol on Jan. 6, 2021, then entered the building that afternoon, court documents say.

Last month, he pleaded guilty to one federal charge of disorderly conduct inside a restricted building. His sentencing is scheduled for September.

An attorney representing Ayres declined to comment when contacted by ABC News.