Federal prosecutors in Greenbelt, Maryland said that US citizens Brian Lemley, 33, and William Bilbrough, 19, and Canadian Patrik Jordan Mathews, 27, were all charged with felony firearms violations.

Lemley and Bilbrough were also charged in relation to transporting and harboring an illegal alien.

Prosecutors said the three were members of The Base, described as an international network of white nationalists who have paramilitary training camps and who discuss online bomb-making and "committing acts of violence against minority communities."

The arrest came a day after the governor of the neighboring state of Virginia declared a "state of emergency" ahead of a gun rights rally in the capital of Richmond, citing "credible threats" of violence from white nationalist and militia groups.

Mathews, a Canadian army reserve combat engineer trained in explosives, was reported missing in Canada in August 2019 after he was suspended from his reserve unit in relation alleged neo-Nazi activities.

The FBI said he illegally crossed into the United States and was met in Minnesota by Lemley and Bilbrough, who drove him to Maryland.

In January, FBI investigators observed them assembling from parts an assault rifle and test-firing it at a Maryland shooting range at rates of more than one round at a time, making it an illegal automatic firearm.

"Oops, it looks like I accidentally made a machine gun," Lemley told Mathews, according to court filings.

After testing the gun multiple times last month, the group ordered 1,500 rounds of ammunition.

They also had plate carriers, a component of body armor.

The indictment did not say what the three had planned for the assault rifle and ammunition.

US media cited unnamed law enforcement officials as saying the three discussed going to the Richmond protest Monday.

The protest is against a new Virginia law banning guns in the buildings of the state legislature.

Law enforcement has expressed strong concerns over potential violence at the rally, where many protestors could carry firearms.

"Law enforcement intelligence analysts have identified credible threats of violence surrounding the event, along with white nationalist rhetoric and plans by out-of-state militia groups to attend," Virginia Governor Ralph Northam said Wednesday in a statement.

Lemley and Bilbrough face up to 10 years in prison for conspiracy to harbor an alien.

Additionally, Lemley and Mathews each face up to 20 years in prison on two separate firearms-related charges.