A man believed to be an online acquaintance of accused synagogue shooter Robert Bowers was arrested after he posted on a social media site that the massacre "was a dry run" and that "there was more to come," according to an FBI affidavit.

The alleged statements themselves didn't prompt formal allegations, but the U.S. Attorney in Washington charged Jeffrey R. Clark Jr. with transportation of a firearm across state lines and possession of illegal, high capacity magazines intended for use with AR-15 weapons.

The case against Clark Jr., 30, was filed Friday but unsealed Tuesday. The Metropolitan Police Department assisted in the arrest in the Bloomingdale neighborhood of Washington, D.C., and described Clark in its public incident report as a "white supremacist" involved in a "suspected hate crime" investigation.

The FBI affidavit, filed in support of the U.S. Attorney's criminal complaint, alleges that two family members called the FBI after becoming concerned about Clark's behavior, which they described as "really riled up" and "agitated."

They believed he was "heavily involved" in the alt-right movement, according to the FBI.

Agents said that Clark and his younger brother attended the "Unite the Right" rally in Charlottesville, Virginia, last year. The gathering of white nationalists and alt-right supporters ended in one death and much violence.

Family members told the FBI they believe the brothers had photos from the event with James Alex Fields, the man prosecutors say drove a car into a crowd of protesters, killing 32-year-old Heather Heyer and injuring 19 other protesters, the filing states.

The witnesses also told agents that the pair "admired" Timothy McVeigh, Ted Kaczynski, and Charles Manson," the affidavit staes. The two believed a race war was coming, "and they wanted to expedite it," according to the FBI's account of what relatives said.

The bureau alleged that Clark once said he and his late brother fantasized about killing "Jews and blacks."

Additionally, the FBI stated, Clark's adult younger brother committed suicide within three hours of the synagogue attack, which claimed the lives of 11 worshipers. After his sibling's death Clark told family members he believed Bowers was a friend on conservative social media platform Gab, according to the filing.

According to the FBI, Bowers was a self-proclaimed white supremacist upset that a Jewish organization, the Hebrew Immigrant Aid Society, or HIAS, has long supported Central American refugees.

The violence came at the height of President Donald Trump's midterm election campaigning, during which he often raised the specter of an "invasion" of Central Americans participating in a migrant caravan headed for the southern border.

The legal document lists what prosecutors say are some of Clark's postings and musings, including a Gab rant that stated the victims of the synagogue attack, which federal prosecutors allege was a hate crime, "were all active supporters of pedophilia … and every last one of them deserved exactly what happened to them and so much worse."

Clark said that the synagogue violence was justified because, he believed, "a homosexual Jewish couple was having an adopted baby circumcised that week," according to the court filing.

He called Bowers a "hero," according to the FBI.

Clark's family thought he was smoking marijuana and possibly methamphetamine. On Gab, the Washington resident described himself as a "Meth-Smoking, Pipe Bomb making, mailman-murding,#Fed,#DemoKKKrat, Che Guevara of the altright," according to the affidavit.

Concerned that he could be a danger to himself or others, the family members made a failed attempt to take Clark's firearms sometime before alerting authorities, the filing said.

During a search of Clark's home Friday, agents seized a shotgun, a rifle and a handgun — all registered to Clark and his brother — as well as an unregistered Colt .38 Special that a relative handed over, according to the allegations.

"In addition," the FBI affidavit states, "agents recovered four high-capacity AR-15 magazines capable of holding up to 30 rounds of ammunition."

Washington, D.C. outlawed such magazines.

Clark lives in Washington with his father and sister, the affidavit states. NBC News reached out to his public defender but did not get an immediate response.