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Federal investigators were watching earlier this month as two men accused of being white supremacists packed cases of food and supplies and allegedly discussed ways to attack a gun rights rally in Richmond, Virginia, according to court documents filed Tuesday.

Brian Mark Lemley Jr. and Patrik Jordan Mathews, who were arrested at their Delaware apartment in a pre-dawn raid last Thursday, had allegedly built “a functioning assault rifle” and amassed rounds of ammunition ahead of Monday’s rally, which they hoped would mark the opening of a race war, according to the documents.

The documents, a 29-page detention memo filed Tuesday by prosecutors in Maryland, provide more details about the case being built against Lemley, Mathews, and William Garfield Bilbrough IV — three defendants accused of being part of a neo-Nazi group known as The Base.

The memo provides new insight into the evidence the FBI had gathered when they decided to move on the men ahead of the rally, and how they got that evidence: a hidden camera and recording device planted in their house.

The men are charged with firearms and immigration-related offenses and are being held without bail ahead of detention hearings on Wednesday.

In one plan described in the memo, the men would derail trains and sabotage power lines in Virginia “in order to bring the economic collapse” of the US. In another scheme, they would use a special scope on their gun to ambush civilians and police officers from a distance.

“I literally need, I need to claim my first victim,” Lemley said, according to the documents.

Mathews, the documents say, told Lemley that “we could essentially like be literally hunting people,” and said, “you could provide overwatch while I get close to do what needs to be done to certain things.”

“We can’t let Virginia go to waste, we just can’t,” Mathews said in regard to the Richmond rally, the memo says. “You know what, Virginia will be our day.”

Lemley and Mathews later changed their plans away from the Richmond rally, deciding to stay a few counties away where they could respond to any violence that spilled out of the gathering, according to the detention memo.

“We’re loading the truck for the war,” Lemley said, according to the memo, “but we’re not going into a place where we’re going to get arrested. We’re gonna stay on the outskirts.”

The FBI watched as the men packed and listened to their discussions using surveillance devices that were installed last month in their Delaware apartment, the memo says.

Investigators also conducted a “sneak-and-peek warrant” — a court-authorized delayed-notification search — where they discovered rifle cases and “go-bags” with pre-made meals and knives, the memo says.

The detention memo also includes a number of other surveillance photos of the men, underscoring the depth of the FBI’s investigation: as Lemley and Mathews drove to Georgia to the residence of another unidentified Base member, and as Lemley and Mathews picked up 150 rounds of ammunition at a store in Delaware.

The FBI had at least one undercover agent who infiltrated The Base, according to the documents in a related case in Wisconsin and had set up a surveillance camera to monitor Lemley and Mathews’ trip to a shooting range.

Monday’s rally in Richmond was peaceful. Some 22,000 people attended, according to the Virginia Division of Capitol Police, as local law enforcement carried out extreme security precautions, including a ban on firearms on Capitol grounds.

In the days before the rally, authorities across the country arrested seven men associated with The Base, including the three being held in Maryland.

A member of the group arrested in Wisconsin and accused of vandalizing a synagogue there was released Friday after an initial appearance in federal court. Three men arrested in Georgia and charged with murder conspiracy in state court are behind bars without an opportunity for bail.

Lemley and Mathews face weapons charges. Lemley and Bilbrough are charged with crimes related to transporting and harboring Mathews, who was in the United States illegally from Canada, according to charging documents.

The Justice Department requested that Lemley, Mathews, and Bilbrough remain in jail ahead of trial, citing their risk of flight and danger to the community.

At a hearing last Thursday, Robert Bonsib, an attorney for Bilbrough, said he was “underwhelmed” by the prosecutor’s evidence that Bilbrough, who doesn’t have a passport and is a college student who lives with his grandmother, posed a flight risk.

A federal judge ordered the three men to remain in custody ahead of detention hearings scheduled for this week. Mathews’ and Bilbrough’s hearings are scheduled for Wednesday. Earlier Tuesday, Lemley withdrew his request for a hearing and consented to detention.

Bonsib declined to comment to CNN on the details from the detention memo Tuesday.

CNN has reached out to attorneys for Lemley. It is not clear who is representing Mathews.

By David Shortell, CNN