Plea agreement negotiations are underway to resolve gun-related charges against a self-described white nationalist from Washington, D.C., whose relatives reported concerns about his behavior after last year's Pittsburgh synagogue massacre, according to a court filing Wednesday.

Federal prosecutors and a public defender representing Jeffrey Clark Jr. have been negotiating since last November and have taken "some substantial steps" that increase the likelihood of a deal, the filing says.

A federal judge agreed to postpone a status hearing for Clark's case from Thursday to May 3, giving attorneys more time.

Clark pleaded not guilty to illegally possessing firearms and high-capacity magazines for rifle ammunition.

The FBI said Clark followed synagogue shooting suspect Robert Bowers on the Gab social media platform and referred to him as a "hero" in a post after the Oct. 27 shootings. Bowers spewed anti-Semitic hatred on his Gab account before killing 11 people inside the synagogue, authorities said.

Clark, then 30, was arrested after relatives called the FBI on Nov. 2 to report their concerns that he could be a danger to himself or others. Relatives told FBI agents that Clark became "really riled up" after his younger brother, Edward, shot and killed himself within hours of the Pittsburgh attack.

"After the death of Edward Clark, Jeffrey Clark became more outspoken about his radical views, expressing them openly to his family members who were in the area following Edward Clark's death," an FBI agent wrote in an affidavit. "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 relatives also 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, the agent wrote.

Jeffrey Clark has remained detained in federal custody since his arrest.

Public defender David Bos has said there's no connection between his client and Bowers. After Clark's arrest, Pittsburgh-based U.S. Attorney Scott Brady's office issued a statement that said there is no evidence that any other individuals besides Bowers "were involved in, or had prior knowledge of" the deadly attack.