A self-described white nationalist with a social media connection to the suspect in last month's Pittsburgh synagogue massacre faces gun-related charges after his relatives reported concerns about his behavior and violent rhetoric to the FBI, according to court records.

Jeffrey Clark Jr., 30, of Washington, D.C., was arrested last Friday after relatives told FBI agents they feared he could be a danger to himself or others. Clark became "really riled up" after his younger brother, Edward, shot and killed himself within hours of the Pittsburgh attack, the relatives told agents.

The relatives believed 23-year-old Edward Clark may have been planning to carry out an "act of violence" that day before he went to Theodore Roosevelt Island in Washington and killed himself, an FBI agent wrote in an affidavit.

"According to (two relatives), Jeffrey and Edward Clark believed that there would be a race revolution, and they wanted to expedite it," the affidavit says.

Clark made his initial court appearance Tuesday on charges that he illegally possessed firearms and high-capacity magazines for rifle ammunition. A federal magistrate ordered Clark held without bond pending a detention hearing Friday.

A federal public defender assigned to represent Clark didn't immediately respond to a call or email Wednesday.

After a gunman killed 11 people at a Pittsburgh synagogue Oct. 27, Clark posted social media messages describing shooting suspect Robert Bowers as a "hero" and warning of a "dry run for things to come," FBI Special Agent Michael Bauknight wrote in his affidavit.

Clark was one of Bowers' followers on Gab, a social media platform that became a haven for far-right extremists who have been banned by Twitter for racist and harassing messages. Bowers posted a slew of anti-Semitic messages on Gab before the shooting. Domain registrar GoDaddy dropped Gab after the shooting, knocking it offline for several days.

Jeffrey Clark became more outspoken about his "radical views" in his relatives' presence after his brother killed himself, the FBI affidavit says.

"During these conversations, Jeffrey Clark defended Robert Bowers' killings at the Tree-of-Life Synagogue. Jeffrey Clark also stated that he and Edward Clark had both fantasized about killing 'Jews and blacks,'" the agent wrote.

Clark told FBI agents who arrested him last Friday that he was a member of white nationalist groups "and followed their ideology," a court filing says. He also said he and his brother became interested in guns after the 2016 presidential election "because they believed there was going to be a civil war," the filing adds.

The brothers attended the "Unite the Right" white nationalist rally in Charlottesville, Virginia, that erupted in violence in August 2017. Their relatives believe the brothers were photographed standing next to James Alex Fields, the man charged with plowing his car into a crowd of Charlottesville counterprotesters and killing one of them, Heather Heyer.

Jeffrey Clark's Gab username was "DC Bowl Gang," an apparent reference to the haircut style of Dylann Roof, the white supremacist who shot and killed nine black people in 2015 at a church in Charleston, South Carolina.

Clark's relatives told the FBI he abused drugs. On his Gab page, he described himself as, "Meth-Smoking, Pipe Bomb making, mailman-murding,#Fed,#DemoKKKrat, Che Guevara of the altright." The "alt-right" is a fringe movement that's a loose mix of white nationalism, anti-Semitism and other far-right extremist views.

Clark told FBI agents he had contact with Bowers on Gab but became "evasive" when asked about those conversations, a court filing says. Clark also was reluctant to talk about his brother but told agents he believed his suicide had nothing to do with Bowers, the filing adds.