A 46-year-old man armed with an assault rifle opened fire at a synagogue in Pittsburgh's Squirrel Hill neighborhood on Saturday morning, the Jewish Sabbath, killing 11 people and injuring four police officers, according to officials and law enforcement sources.

The suspect, Robert Bowers, traded gunfire with police and was shot several times. Bowers, who was in fair condition at a hospital, was charged late Saturday with 29 federal counts, including hate crimes and weapons offenses. It wasn't immediately known if Bowers has an attorney to speak on his behalf.

A circumcision celebration known as a bris was taking place at the Tree of Life synagogue in Squirrel Hill when gunfire erupted, congregants told MSNBC. Officials said the shooting, which officers responded to just before 10 a.m., is being investigated as a hate crime.

"Please know that justice in this case will be swift and it will be severe," Scott Brady, the chief federal prosecutor in western Pennsylvania, said at a news conference, characterizing the slaughter as a "terrible and unspeakable act of hate."

"Today, the nightmare has hit home in the city of Pittsburgh," said public safety director Wendell Hissrich, who described the "very horrific crime scene" as "one of the worst that I've seen."

No children were killed in the attack, officials said. There were six injuries, including the four male officers, three of whom were shot. The officers sustained serious but non-life-threatening injuries while the others were critically wounded, and they were all taken to area hospitals.

The Anti-Defamation League, which tracks anti-Semitism in the United States, said in a statement this "is likely the deadliest attack on the Jewish community in the history of the United States."

Authorities say that just before 10 a.m., Bower entered the large synagogue with an assault-style rifle and three handguns.

Witnesses said the gunman shouted anti-Semitic comments as he was being taken into custody.