The morning of the synagogue massacre in Pittsburgh last month, 23-year-old Edward Clark killed himself in a Washington, D.C., park.

Ever since, the authorities have been piecing together a disturbing portrait of Clark and his older brother, Jeffrey Clark, 30, who had been online friends with the suspect in the Pittsburgh attack. Online, Jeffrey Clark had called the massacre a “dry run for things to come.”

Now, there are new indications that the lives of the Clark brothers in Washington intersected with some of the most violent white supremacist groups in the country — including Atomwaffen Division, a neo-Nazi organization that calls for racially motivated, lone-wolf terror attacks like Pittsburgh and whose members or associates have been charged in five murders in the last two years.

In court filings, prosecutors say Edward Clark had used the handle DC_Stormer on the social media platform Gab, a site favored by racists and other extremists. That same handle appears in Atomwaffen’s secret chat logs, which ProPublica and Frontline have been reviewing as part of ongoing reporting about the resurgence of white supremacists and the groups involved in the violent 2017 rally in Charlottesville, Virginia.

Both Clark brothers participated in the rally, according accounts given to prosecutors by their family, and were affiliated with Vanguard America, a white supremacist group at the center of the violence that day. Many Vanguard America members ended up joining Atomwaffen in the months after Charlottesville.

In the Atomwaffen chat logs obtained by ProPublica and Frontline, members of the group from Northern Virginia mention DC_Stormer and their plans to see one of the group’s leaders in Houston.

A former Atomwaffen member told Frontline and ProPublica on Wednesday that he is familiar with the handle DC_Stormer, although he is not certain it belongs to Edward Clark. He said DC_Stormer was most likely either a member or what the group called an “initiate,” someone seeking formal entrance into the group.

Last week, authorities in Washington arrested Jeffrey Clark on weapons charges and made public his online connection to the Pittsburgh suspect, Robert Bowers. Federal prosecutors, in court filings, said that he may have known more about the attack and that both brothers had talked about wanting to kill Jews and blacks.

Requests for comment made to Jeffrey Clark’s lawyer via telephone and email were not immediately returned. He has not yet entered a plea.

The FBI is investigating Atomwaffen and its members, but it would not comment on Wednesday when asked if the bureau had reason to believe either Clark brother had been associated with the group.

In reporting for an upcoming film, “Documenting Hate: New American Nazis,” which airs on Tuesday, Frontline and ProPublica developed a list of at least 80 members or associates of Atomwaffen — some with their names, most with their online handles. Based on interviews with former members and the chat logs, at least seven current or former Army troops or Marines have joined the group since its formation.

An FBI agent stands behind a police cordon outside the Tree of Life Synagogue in Pittsburgh after a shooting there in October. (Brendan Smialowski/AFP/Getty Images)

The group came together in Florida in 2015 and draws much of its inspiration from the writings of an obscure neo-Nazi, James Mason, who advocates lone-wolf attacks and reveres figures such as Oklahoma City bomber Timothy McVeigh and notorious killer Charles Manson.

The Clark family told prosecutors that the brothers had also openly admired McVeigh, Manson and Unabomber Ted Kaczynski. They said the brothers believed that there would be a race revolution, and they wanted to hasten it.

In a variety of online posts, the Clark brothers are often pictured together, sometimes with prominent figures in the white supremacist movement. In the court filing, prosecutors said the two appeared in photos with weapons, masks and a flag with a skull and crossbones. One of the pictures included an image of Dylann Roof, the young man who killed nine black church members in South Carolina.

According to the authorities, on the day of the Pittsburgh killings, Jeffrey Clark “engaged in unusual behavior” when he learned his brother, Edward, wasn’t home and told his mother he was going to call the police. Edward Clark was eventually found on Theodore Roosevelt Island, dead of a gunshot from a Beretta pistol, one of two guns registered to him. The police found magazines with additional ammunition at the scene, but no suicide note.

It wasn’t until days later that family members contacted the FBI office in Washington and expressed worry that Jeffrey Clark might pose a danger to himself or others. The family told the FBI of the men’s associations with white hate groups and said they feared Edward Clark might have been intending to use the extra ammunition to kill others.

The family members said Jeffrey Clark had been “really riled up” and “agitated” since the Pittsburgh murders and said he openly worried the FBI might be coming to see him because of his connection to Bowers.

Priyanka Boghani and Karim Hajj contributed reporting.