As jurors spent the summer in a Pittsburgh courthouse listening to weeks of testimony about the massacre that left 11 dead in a neighborhood synagogue, Hardy Carroll Lloyd was busy.

He placed Nazi-themed stickers on street signs and playground equipment around the city, federal authorities said, and his hate-filled screeds arrived in the email inboxes of local reporters and Jewish organizations. Photos of people who had testified at the trial, including those who had been wounded in the synagogue attack, showed up on an “enemies list” on his website.

On social media, he pledged to track down the jurors at the trial, sought guidance on making ghost guns and pipe bombs, and urged anyone reading his website to follow the example of Robert Bowers, the man who carried out the synagogue massacre, the deadliest antisemitic attack in the country’s history.

On Thursday morning, a week after Mr. Bowers was sentenced to death, Mr. Lloyd was arrested by federal agents in Follansbee, W.Va., a tiny city about 40 miles from the federal courthouse in Pittsburgh. He was charged with three federal criminal counts, including witness tampering and obstruction of justice, in connection with what prosecutors say was a campaign of threats against people involved in the trial.

“Jury trials are a hallmark of the American justice system and attempts to intimidate witnesses or jurors will be met with a strong response,” William Ihlenfeld, the U.S. attorney for the Northern District of West Virginia, said in a statement. “The use of hateful threats in an effort to undermine a trial is especially troubling.”

The arrest of Mr. Lloyd, 45, whose antisemitic missives have tormented the Jewish residents of Pittsburgh for years, was met with relief by his targets, some of whom had been cooperating with law enforcement. But it was a bleak reminder that in the sea of white supremacist content online, there was nothing singular about the violent bigotry spewed by Mr. Bowers.

“Bowers was on nobody’s radar,” said Bradley Orsini, a former F.B.I. agent who is now a senior adviser with the Secure Community Network, a nonprofit that works with Jewish communities across the country on safety and protection. “Lloyd was different,” he said. Out of the roughly 1,500 potential threats that the network regularly tracks online, Mr. Orsini said that Mr. Lloyd was “in our Top 10.”

A message left for a federal public defender assigned to him was not returned.

Mr. Orsini said that Mr. Lloyd’s zeal for publicly identifying and threatening such a wide array of people — trial participants and local officials, but also everyday Pittsburgh residents who expressed a distaste for his racist fliers — set him apart. And unlike the vast majority of other people spouting bigotry online, Mr. Orsini added, Mr. Lloyd had actually killed someone.

In 2004, Mr. Lloyd fatally shot a woman in Pittsburgh. He asserted self-defense and was acquitted at trial, though he has since boasted of the killing and taunted the woman’s family online.

By that time, Mr. Lloyd, who grew up near Squirrel Hill, the Pittsburgh neighborhood that is the center of Jewish life in the city and home to the synagogue that Mr. Bowers attacked, had already become involved with high-profile white supremacists. In an interview with The Pittsburgh Post-Gazette in 2001, Mr. Lloyd’s father, a doctor, said that his son had “become fixated on race issues as a child” and attributed this to a developmental disorder. A message left for his father on Thursday was not returned.

For much of the last 15 years, Mr. Lloyd was in and out of prison serving a sentence on a gun conviction that kept stretching out; multiple times, he was let out, only to be sent back for violating the terms of his release, including urging violence against Jews.

In 2020, he reached the end of his most recent sentence. He moved to Texas, but in 2022 was declared a wanted man by law enforcement for making terroristic threats. He then went mostly quiet — until the Bowers trial began.

“It was right around that time that the behavior escalated,” said Shawn Brokos, the security director for the Jewish Federation of Greater Pittsburgh. As the trial went on, Mr. Lloyd’s rhetoric ratcheted up, she said, and it was clear that he was once again living close by. According to an affidavit submitted in court by a federal agent this week, law enforcement officials had been following his whereabouts and activities.

But making an arrest during Mr. Bowers’s trial could have complicated the case, said Bruce Antkowiak, a law professor at Saint Vincent College, near Pittsburgh. “It would have presented a considerable conundrum for the judge,” he said, possibly infecting the jury deliberations and even raising the prospect of a mistrial.

The authorities kept watching. On Saturday, Mr. Lloyd posted a message for Pittsburgh on his Telegram channel: “Well, we warned you, Filthburg. We told you to release this great Warrior or Pgh would pay the price. Pgh shall pay the price.”

Five days later, on Thursday morning, he was arrested. He is to remain in custody ahead of a hearing on Tuesday.