At least 11 people are dead and six injured after a man armed with an assault rifle and multiple handguns opened fire on a Shabbat service Saturday morning at a synagogue in Pittsburgh, Pennsylvania, in what authorities describe as a federal hate crime.

Police responded to 911 reports of an active shooter at the Tree of Life Synagogue in the Squirrel Hill neighborhood of Pittsburgh at around 10 a.m. where three consecutive congregations had just started. One of the congregations was holding a bris, a ceremony which typically takes places on the eighth day of a newborn boy’s life.

By 11.20 a.m., Pittsburgh police told reporters one suspect was in custody, but it was still a “dangerous situation,” urging people to stay away from the synagogue.

State and local officials held an emotional press briefing at 4.00 p.m. Public safety director Wendell Hissrich said that no children were among the 11 victims. Four police officers were reportedly shot during a gunfight with the suspect. Two additional people were wounded. One of those shot is in good condition, the other, a 70-year-old man who was shot multiple times in his torso, is in critical condition.

“This is the most horrific crime scene I’ve seen in my 22 years in the FBI,” Bob Jones, the special agent in charge of the FBI's Pittsburgh office, said. “Members were brutally murdered by a gunman targeting them simply because of their faith.”

Jonathan Greenblatt, CEO of the Anti-Defamation League (ADL), said in a statement: “It is simply unconscionable for Jews to be targeted during worship on a Sabbath morning, and unthinkable that it would happen in the United States of America in this day and age. Unfortunately, this violent attack – the deadliest anti-Semitic attack in the United States since 2014 – occurs at time when ADL has reported a historic increase in both anti-Semitic incidents and anti-Semitic online harassment. “

Officials have identified the suspect as Robert Bowers, 46, of Pittsburgh. He is being treated for multiple gunshot wounds, but is in fine condition officials said. Citing police sources, KDKA said that Bowers shouted “all Jews must die” before opening fire.

An archived version of what's believed to be the suspect’s Gab page shows regular posting of virulently anti-semitic memes. The account posted hours before the shooting: “HIAS [Hebrew Immigrant Aid Society] likes to bring invaders in that kill our people. I can’t sit by and watch my people get slaughtered. Screw your optics, I’m going in.”

Gab, a social network that's regularly accused of being a platform for the alt-right, released a statement condemning the shooting and said they backed up user data from Bowers’ account before taking it offline. The company also said it had contacted the FBI.

Officials said that Bowers was not previously known to law enforcement and is believed to have acted alone.

Scott Brady, U.S. Attorney for the Western District of Pennsylvania, said his office is putting all their resources into the case, and that his office might be ready to file a criminal complaint as early as today. “The actions of Robert Bowers represents the worst of humanity,’ Brady said.

Squirrel Hill is known for having a large Jewish community. The shooting reportedly took place in the basement and on the main floor. The suspect was apprehended on the third floor.

Former Tree of Life rabbi Chuck Diamond told CNN that the doors would have been open during Shabbat services. "On Shabbat, the doors are unlocked," he said.

Michael Eisenberg, former president of the Tree of Life, told CNN that he doesn’t recall the synagogue receiving threats. He also said that security at the building would have likely been minimal.

“The Shabbat is a time for reflection, it is a time for finding peace, not for violence,” Pennsylvania Gov. Tom Wolf said at Saturday afternoon’s press conference. “We cannot accept this violence as a normal part of American life.” In comments before boarding a plane to Indianapolis, President Donald Trump called the shooting “pure evil” and anti-semitic. Trump also said that if the synagogue had had an armed guard inside, "maybe it could have been a different situation."