FBI agents on Thursday arrested a former Canadian Armed Forces reservist and two other men who are linked to a violent white supremacist group and who were believed to be heading to a pro-gun rally next week in Virginia's capital.

The three men are members of The Base, a neo-Nazi group, and were arrested on federal charges in a criminal complaint unsealed in Maryland, according to a United States Department of Justice news release.

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Tuesday's complaint charges Canadian national Patrik Jordan Mathews, 27, and Brian Mark Lemley Jr, 33, of Elkton, Maryland, with transporting a firearm and ammunition with intent to commit a felony. William Garfield Bilbrough IV, 19, of Denton, Maryland, is charged with "transporting and harboring aliens".

The three men were believed to be planning to attend the pro-gun rally planned for Monday in Richmond, according to a law enforcement official who spoke to The Associated Press on condition of anonymity to discuss an active investigation.

In encrypted chat rooms, members of The Base have discussed committing acts of violence against black people and Jews, ways to make improvised explosive devices, their military-style training camps and their desire to create a white "ethno-state", according to an FBI agent's affidavit.

Mathews and Lemley were arrested in Delaware and Bilbrough was arrested in Maryland, according to Marcia Murphy, a spokeswoman for the US Attorney's office in Maryland. All three men were scheduled to make their initial court appearances Thursday afternoon in Greenbelt, Maryland.

Court papers say Mathews crossed the US border between official ports of entry near Minnesota in August and investigators allege Lemley and Bilbrough then drove from Maryland to Michigan to pick up Mathews before the three headed to Maryland in late August.

Mathews was a combat engineer in the Canadian Army Reserve. Lemley was a "cavalry scout" in the US Army, according to a court filing.

The search

US and Canadian authorities had been searching for Mathews after his truck was found in September near the border between the two countries. He was last seen by family members in Beausejour, northeast of Winnipeg, on August 24, according to the Royal Canadian Mounted Police. The Canadian military's intelligence unit was investigating Mathews for "possible racist extremist activities" for several months, according to the Canadian Department of National Defence.

Authorities say Lemley and Mathews built an assault rifle using several parts, including an upper receiver that Lemley had ordered and shipped to a Maryland home. In December, the three men gathered at an apartment that Lemley and Mathews rented in Delaware, where they discussed The Base and its activities and members, passed around the assault file and tried to make the drug DMT, a hallucinogen, according to court papers.

A few days later, Lemley and Mathews bought 150 rounds of ammunition and paper shooting targets and Lemley was spotted by an FBI agent at a gun range in Maryland. Court papers say federal agents heard the gun firing in rapid succession and authorities allege that Lemley later told Mathews: "Oh oops, it looks like I accidentally made a machinegun."

Federal agents appeared to be tracking the men's movements and set up a stationary camera near the gun range, which captured video of Mathews shooting the gun there on January 5. Court documents say Lemley had also ordered 1,500 rounds of ammunition and he and Mathews visited the gun range as recently as Saturday.

Lemley also is charged with transporting a machinegun and "disposing of a firearm and ammunition to an alien unlawfully present in the United States."

The Anti-Defamation League said members of The Base and other white supremacist groups have frequently posted online messages advocating for "accelerationism", a fringe philosophy in which far-right individuals "have assigned to their desire to hasten the collapse of society as we know it".

"The term is widely used by those on the fringes of the movement, who employ it openly and enthusiastically on mainstream platforms, as well as in the shadows of private, encrypted chat rooms," the ADL said.

In recent months, FBI agents have arrested several members of a different far-right group, the neo-Nazi Atomwaffen Division. Atomwaffen has been linked to several killings, including the 2017 shooting deaths of two men at an apartment in Tampa, Florida.