PITTSBURGH—Robert Bowers was a virtual unknown to police, and to most of his neighbors, until he pulled up outside Pittsburgh’s Tree of Life synagogue on Saturday, heavily armed and ready to commit what federal prosecutors have called an “unspeakable act of hate.”

In a shooting spree that lasted roughly 20 minutes, prosecutors say, Bowers gunned down 11 people attending a baby-naming ceremony on the Sabbath, and wounded six others, including four police officers. The carnage left behind stunned seasoned investigators and made Bowers a candidate for the death penalty.

The victims, all members of the congregation, ranged in age from 54 to 97. Many of them were elderly and in some cases would not have been able to react quickly enough to escape.

Authorities, court records and witnesses offered a gut-wrenching account of what took place when Bowers—who forecast the attack on social media shortly before he pulled up to the building in a teal Hyundai—allegedly committed what the Anti-Defamation League says is the deadliest attack ever on Jews in America.

Pittsburgh Mayor Bill Peduto described the attack as the “darkest day of Pittsburgh’s history” and said the city could only recover by coming together.

“We have been knocked down before,” he said. “But we’ve always been able to stand back up because we worked together.”

E. Joseph Charny, a 90-year-old survivor of the mass shooting, told The Washington Post that many congregants had their heads bowed when the gunman appeared in the doorway and began firing.

He said he couldn’t recall Bowers saying anything. He looked up after gunshots rang out and “there were all these dead bodies.”

“ At first I felt numb, then thankful. ” — Shooting survivor Joseph Charny

Charny said he ran with two others, a rabbi and his assistant, and they hid in a third-floor storage room while the shooter went from room to room on the other end of the building.

“At first I felt numb, then thankful,” he told the Post. “I don’t need to tell you how terrible this has all been.”

The killer did not give his targets much of a chance.

He entered the front door of the conservative synagogue in the largely Jewish enclave on the outskirts of Pittsburgh sometime around 9:45 a.m., when police received the first call about shots, and began firing dozens of bullets, switching between a Colt AR-15 semi-automatic rifle and three Glock .357 handguns.

Bowers, who was exiting the building as officers arrived, continued firing as police officers confronted him striking two before retreating inside the building, authorities said. He is accused of critically wounding at least one member of a SWAT team and injuring three others who chased him up to the third floor of the building, where he opened fire again.

Two of the SWAT officers were shot multiple times, and Bowers himself was also injured in the exchange of gunfire, according to authorities. He was in “fair” condition with multiple gunshot wounds as of Sunday morning and under federal custody at a local hospital. He is expected to appear in federal court in Pittsburgh Monday to face 29 criminal counts, including charges of murder and weapons possession.

Of the four officers injured, one has been released from the hospital and another is expected to be released Sunday. Two others, however, remain hospitalized and face further treatment, police said Sunday.

With the city of Pittsburgh still reeling from the attack and many across the country asking what prompted such a violent rampage, community leaders say the answer is simple: Bower was motivated by hate.

The federal complaint against Bowers says as much in describing his words immediately after the attack as expressing “an animus toward people of the Jewish faith.”

“ They're committing genocide to my people. I just want to kill Jews. ” — Synagogue shooting suspect Robert Bowers

According to court records, he told one officer, “They're committing genocide to my people. I just want to kill Jews.” He repeated comments regarding genocide, his desire to kill Jewish people, and that Jewish people “needed to die.”

Eyewitnesses also have alleged that he screamed, “All Jews must die” upon entering the synagogue.

It remains unclear why the Tree of Life synagogue was singled out, but social media posts linked to Bowers—who had no criminal record—show a deep-seated anti-Semitism.

In a post on the social media site Gab just hours before the alleged massacre, Bowers referenced the Hebrew Immigrant Aid Society, a Jewish organization that offers assistance to refugees, ranting about “invaders” being brought in to “kill our people.”

“I can’t sit by and watch my people get slaughtered. Screw your optics,” he wrote under the handle @onedingo. “I’m going in.”

In an earlier post, he appeared to suggest a Jewish conspiracy was behind sexual assault allegations against then-Supreme Court nominee Brett Kavanaugh, questioning whether one of Kavanaugh’s accusers was Jewish.

Police say Bowers is believed to have acted alone and did not have plans to target any other sites, although FBI Special Agent in Charge Robert Jones said he had no doubt that if officers had not taken the suspect down inside the synagogue, there would have been “additional violence.”

“ It’s scary how normal he was. ” — Kerri Owens, neighbor of shooting suspect

Neighbors interviewed by The Daily Beast said Bowers never raised any red flags.

“It’s scary how normal he was,” said 30-year-old Kerri Owens, who lived next door to Bowers in a suburban apartment complex called McAnulty Acres in the suburb of Baldwin.

“I wish there were warning signs,” neighbor Chris Hall said.

Bowers is said to have stayed home a lot and never had many visitors. Hall described him as a “very normal, unremarkable guy.”

He had worked as a trucker for B. Keppel Trucking, a Pennsylvania company owned by Robert Keppel. Keppel has already spoken to the FBI but has declined to comment, his wife told The Daily Beast.

Victoria Bekiempis, Adam Rawnsley and Tracy Connor contributed reporting