(CNN) The FBI arrested three alleged members of a white supremacist group early Thursday, including two men accused of possessing a machine gun, over 1,000 rounds of ammunition and body armor parts, according to the Justice Department.

The three were arrested at residences in Delaware and Maryland and taken into custody without incident, FBI spokesman Dave Fitz said.

The men, who the Justice Department says are members of the international white supremacist group known as The Base, were believed to be planning to attend a pro-gun rally in Virginia's capital of Richmond on Monday that is expected to draw a significant crowd of extremists, according to a law enforcement official.

They're charged with multiple firearms and immigration-related offenses and at court appearances Thursday afternoon, a federal judge ordered that the men remain in custody ahead of a detention hearing next week.

Brian Mark Lemley Jr., 33, is accused of transporting a machine gun, as well as transporting a firearm and ammunition with intent to commit a felony.

Lemley and William Garfield Bilbrough IV, 19, are also accused of transporting and harboring an alien -- 27-year-old Patrik Jordan Mathews, a Canadian citizen and former combat engineer in the Canadian Army Reserve.

Brian Lemley, Patrik Mathews and William Bilbrough had initial appearances before US Magistrate Judge Charles B. Day in US District Court in Greenbelt, Maryland, on Thursday, January 16.

Like Lemley, Mathews is charged with transporting a firearm and ammunition with intent to commit a felony. Mathews is also charged with being an alien in possession of a firearm and ammunition.

The Washington Post first reported the arrests.

Canadian illegally entered US in August, authorities say

Mathews is accused of illegally crossing into the US -- specifically, Minnesota -- from Canada on August 19.

Federal authorities in Canada had reported Mathews as a missing person as of late August. Royal Canadian Mounted Police said in September that they had found a car belonging to Mathews abandoned at a rural property in the border town of Piney, Manitoba.

A Winnipeg newspaper ran a story over the summer alleging Mathews was recruiting for The Base. A spokesman for Canada's Department of National Defence told CNN Mathews was relieved of his military duties in early August as a result of the "seriousness" of allegations against him "and the risk to unit morale and cohesion."

On August 30, Lemley and Bilbrough allegedly drove some 600 miles to pick up Mathews in Michigan, according to the complaint filed in the Maryland federal court.

Court appearances Thursday

Lemley and Mathews, who were arrested together in Delaware, smashed their cell phones and dropped them in the toilet before they were taken into custody, a prosecutor alleged in a hearing Thursday.

At brief appearances in Maryland federal court, the men were ordered to remain in custody ahead of a detention hearing set for Wednesday.

The third man arrested Thursday, Bilbrough, will also remain in custody ahead of a Wednesday detention hearing after his attorney lost a challenge to the prosecutor's claims that Bilbrough posed a "serious risk of flight" who could attempt to obstruct justice if he were to be set free.

Prosecutors showed this screenshot from a recuitment video for The Base in court Thursday.

At Bilbrough's hearing, prosecutors displayed photos that they said showed him standing in a line of men -- all alleged members of The Base at a training camp -- with long guns raised. Bilbrough was identifiable in a close-up photo by his short red hair, prosecutors said.

Many of the alleged members of the group are unknown to law enforcement, prosecutor Thomas Windom said, and Bilbrough could turn to any number of them for "safe harbor."

Bilbrough has also traveled extensively throughout the country to The Base training camps and expressed a desire to fight overseas, Windom said.

After he was arrested early Thursday, Bilbrough told authorities he wanted to travel to Ukraine to fight alongside his nationalist "friends" there, Windom said.

The three men appeared in T-shirts and pants, with Lemley wearing dark pajama bottoms and Mathews in camouflage-patterned pants. Mathews had a bushy beard and unkempt hair.

Court-appointed attorneys represented the three men at separate hearings. Attorneys for Lemley and Mathews did not challenge their continued detention ahead of the hearing next week after prosecutors explained they made the request because the men faced firearms-related charges.

Bilbrough faces charges of aiding and abetting and transporting and harboring an alien after he allegedly helped drive and put up Mathews on the East Coast. Bilbrough shook his head and gave a look of disbelief after judge Charles B. Day said that he was being accused of harboring an alien.

Windom described how Lemley and Mathews allegedly destroyed their phones as he sought to argue that Bilbrough posed a risk of destroying evidence. Windom also alleged that in December, the three men burned a Bible and a number of other unknown objects in the woods of Delaware.

An attorney for Bilbrough, Robert Bonsib, said he was "underwhelmed" by the prosecutor's evidence that Bilbrough posed a flight risk.

Bilbrough doesn't have a passport and is a college student who lives with his grandmother, Bonsib said.

Two relatives of Bilbrough sat emotionless in court throughout his hearing. They declined to comment to CNN.

Weapon allegations

According to court documents, in November Lemley ordered a gun part online, and last month, along with Mathews, built a functioning assault rifle using that part.

Lemley and Matthews allegedly purchased some 1,650 rounds of ammunition earlier this month, and practiced using the assault rifle at a gun range in Maryland, where FBI agents had set up a hidden camera to secretly record them as they practiced shooting, according to a criminal complaint.

The Base describes itself as an international network that is training its members to fight in a race war, according to the Counter Extremism Project

Inside encrypted chat rooms, members of the group have discussed creating a white ethno-state and attacking African Americans and Jewish people, according to the criminal complaint.

Group members have also talked about ways to build bombs and the military-style training camps the group runs, the complaint says.

The FBI had been keeping close surveillance of the men for weeks. Conversations the men had inside private residences -- including about making drugs and the assault rifle they built -- are detailed in the complaint.

"Oh oops, it looks like I accidentally made a machine gun," Lemley told Mathews on January 2, according to the complaint.

Earlier this week, Virginia's governor declared a temporary state of emergency around the Monday rally that banned all weapons on state Capitol grounds, citing credible threats of violence.

"State intelligence analysts have identified threats and violent rhetoric similar to what has been seen before other major events such as Charlottesville," Gov. Ralph Northam said, referring to the deadly 2017 white supremacist rally in that Virginia city.

The threats, which are considered credible by law enforcement, come from mainstream channels and alternative dark web channels used by violent groups and white nationalists from outside of Virginia, Northam said.