A gunman who had expressed hatred of Jews exploited that vulnerability, so common in so many houses of worship across the country, in a singularly horrific way.
Armed with a rifle and three handguns, Robert Bowers walked inside the synagogue during Saturday morning worship and began shooting, killing 11 and wounding six before police took him into custody, officials said. It was the deadliest attack on Jews in U.S. history, according to the leader of the Anti-Defamation League.
Bowers traded gunfire with police and was shot multiple times but survived. He was charged late Saturday with 29 federal counts, including weapons offenses and hate crimes. Law enforcement officials planned to discuss the massacre at a news conference Sunday morning.
Four police officers were among the wounded.
The nation's latest mass shooting drew condemnation and expressions of sympathy from politicians and religious leaders of all stripes. With the midterm election just over a week away, it also reignited a longstanding and bitter debate over guns. President Donald Trump said the outcome might have been different if the synagogue "had some kind of protection" from an armed guard, while Pennsylvania's Democratic Gov. Tom Wolf, up for re-election, noted that once again "dangerous weapons are putting our citizens in harm's way."
Calling the shooting an "evil anti-Semitic attack," Trump ordered flags at federal buildings throughout the U.S. to be flown at half-staff in respect for the victims. He said he planned to travel to Pittsburgh, but offered no details.
In the city, thousands gathered for a vigil Saturday night. Some blamed the slaughter on the nation's political climate.
"When you spew hate speech, people act on it. Very simple. And this is the result. A lot of people dead. Senselessly," said Stephen Cohen, co-president of New Light Congregation, which rents space at Tree of Life.
The shooting raised immediate alarm in Jewish communities around the country. Authorities in New York City, Los Angeles, Chicago, Houston, Philadelphia and other cities sent extra patrols to synagogues and other houses of worship.
Little was known about Bowers, who had no apparent criminal record but who is believed to have expressed virulently anti-Semitic views on social media. Authorities said it appears he acted alone.
Worshippers "were brutally murdered by a gunman targeting them simply because of their faith," said Bob Jones, head of the FBI's Pittsburgh office, though he cautioned the shooter's full motive was not yet known.
Scott Brady, the chief federal prosecutor in western Pennsylvania, pledged that "justice in this case will be swift and it will be severe."
It wasn't clear if Bowers had an attorney to speak on his behalf.
The gunman targeted a building that housed three separate congregations, all of which were conducting Sabbath services when the attack began just before 10 a.m. in the tree-lined residential neighborhood of Squirrel Hill, about 10 minutes from downtown Pittsburgh and the hub of Pittsburgh's Jewish community.
The synagogue door was unlocked on the Sabbath "because people are coming for services, and the bell would be ringing constantly. So they do not lock the door, and anybody can just walk in," said Marilyn Honigsberg, administrative assistant for New Light. "And that's what this man did."
Michael Eisenberg, the immediate past president of the Tree of Life, said synagogue officials had not gotten any threats that he knew of before the shooting. But he said security was a concern, and the synagogue had started working to improve it.
"You know, you're always worried that something would happen," said Myron Snider, head of the cemetery committee for New Light. Snider just got out of the hospital on Thursday and missed Saturday's service.
"But you never dream that it would happen like this," Snider added. "Just never ever dream that it would happen like this."
Zachary Weiss, 26, said his father, 60-year-old Stephen Weiss, was inside the synagogue but was unharmed. Weiss said his father told him that he and Tree of Life's rabbi helped congregants take shelter and follow the active shooter response training they'd received months earlier. Stephen Weiss made it out of the building and used a janitor's cellphone to call his family at home.
The attack, his son vowed, "will not define our congregation and will not define our city."