[Read our latest article on Robert Bowers, the suspect in the Pittsburgh shooting.]

For months, Robert D. Bowers had been spewing his anger in post after post on the web, calling immigrants “invaders,” distributing racist memes and asserting that Jews were the “enemy of white people.”

Then, on Saturday, moments before the police say he barged into a Pittsburgh synagogue with an assault rifle and three handguns, he tapped out a final message: “I can’t sit by and watch my people get slaughtered. Screw your optics, I’m going in.”

The authorities said Mr. Bowers, 46, then killed at least 11 people in and around the synagogue, Tree of Life, a spacious building with stained glass windows, a golden memorial tree and a Torah rescued from the Holocaust.

It was the Sabbath, the synagogue’s busiest day. The attack was one of the deadliest on the Jewish community in United States history.

“The actions of Robert Bowers represent the worst of humanity,” said Scott W. Brady, the United States Attorney for the Western District of Pennsylvania. “Please know that justice in this case will be swift and it will be severe.”

Image Robert D. Bowers. Credit... via Agence France-Presse — Getty Images

[A man shouting anti-Semitic slurs opened fire inside a Pittsburgh synagogue where three congregations worshiped.]

The police arrested Mr. Bowers, who had 21 guns registered to his name, according to Representative Mike Doyle of Pennsylvania. Officials said he was not known to law enforcement before the shooting, and a search of the Pennsylvania judiciary database shows only a 2015 traffic violation in his name.

Mr. Bowers took to Gab, a social network that bills itself as a being dedicated to free speech and which is increasingly popular among alt-right activists and white nationalists. After opening an account on it in January, he had shared a stream of anti-Jewish slurs and conspiracy theories. It was on Gab where he found a like-minded community, reposting messages from Nazi supporters.

“Jews are the children of Satan,” read Mr. Bowers’s biography.

[Read more about Gab, a social media platform popular with the far right.]

Mr. Bowers lived about a 25-minute drive south of the synagogue in a brick apartment complex on a dead-end street, where he was frequently spotted smoking cigarettes outside. A neighbor said she could not remember seeing him speak to anyone, not in the two years she’d lived there.