Far-right activist Thomas Caldwell was charged with conspiracy Tuesday, the Justice Department said, over the violent assault on the US Capitol by supporters of President Donald Trump.

 Caldwell, 65, from Virginia, is accused of organizing a group to unlawfully enter the Capitol building in Washington, engage in disorderly conduct and "obstruct, influence, or impede any proceedings before the Congress."

The individuals were mostly members of the Oath Keepers-- a loosely organized far-right anti-government militia, investigators said.

The January 6 storming of the US Capitol bore some of the hallmarks of a spontaneous riot, as a chaotic and disorganized mob of Trump supporters descended on the building.

But still images, videos and communications analyzed since suggest an unsettling level of preparation and organization by some participants, more in line with practices of militia groups that have weapons and guerilla warfare training.

According to the federal investigation, Caldwell, who "appears to have a leadership role within Oath Keepers", was part of the group's "planning and coordinating" efforts to force entry into the US Congress as lawmakers certified the results of the November 2020 election won by Joe Biden.

The charges allege Caldwell helped the Oath Keepers find a hotel to stay in near Washington.

"I strongly recommend you guys get one or two rooms for a night or two," he sent in a Facebook message January 1, according to investigators.

"This is a good location and will allow us to hunt at night if we want to."

The FBI also cites Jessica Watkins and Donovan Crowl, who were arrested in Ohio Monday and also accused of forcibly entering the Capitol, as Caldwell's accomplices

Watkins, 38, a former member of the US military, is involved with an Ohio militia group, according to investigators.

They say she posted photographs and videos on social media of herself dressed in combat gear and participating in the incursion.

"Me before forcing entry into the Capitol Building," Watkins posted alongside a picture on Parler, according to the charge sheet, also boasting about the "Historical Events we created today."

Crowl, a former Marine whom investigators say belongs to the same Ohio militia as Watkins, also admitted his presence at the Capitol on January 6 during an interview to the New Yorker.

He told the magazine he was in Washington to do "security for VIPs" -- whom he did not identify -- and insisted his intentions had been peaceful.

Nearly 90 people have so far been charged -- including five for conspiracy -- in connection with the violence at the Capitol which left five people dead.