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US attorney for western Pennsylvania Scott Brady told a news conference that charges could be filed later on Saturday against the suspected shooter, 46-year-old Robert Bowers from Pittsburgh.
"The actions of Robert Bowers represent the worst of humanity. We are dedicating the entire resources of my office to this federal hate crime investigation and prosecution," Brady said.
The US Department of Justice will file hate crime and other criminal charges against Bowers that could lead to the death penalty, Attorney General Jeff Sessions said.
FBI special agent Bob Jones said the Federal Bureau of Investigation, which is leading the probe into the attack, believed Bowers was acting alone, adding: "We have no knowledge that he was known to law enforcement before today."
KDKA television cited police sources as saying Bowers walked into the building and yelled “All Jews must die.”
Bowers was in a hospital in fair condition with multiple gunshot wounds. Patients at area hospitals included a 61-year-old woman, a 70-year-old man, and a 55-year-old officer. No children were killed, authorities said.
Three police officers were shot and one was injured by shrapnel, authorities said. Two of the six people injured were in critical condition.
RAGING AGAINST 'INVADERS'
A social media post by Bowers on Saturday morning said a Jewish refugee organisation, the 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.”
The comment was posted on Gab, a Philadelphia, Pennsylvania-based social networking service created as an alternative to Twitter. In a statement, Gab.com confirmed the profile belonged to Bowers and also said it had suspended the account and contacted the FBI about it.
The shooting, for which Bowers had an assault rifle and three handguns according to the FBI's Jones, prompted security alerts at houses of worship around the country. It follows a spate of pipe bombs found mailed in recent days to prominent political figures, mostly Democrats including former President Barack Obama.
The Tree of Life synagogue in the city's Squirrel Hill neighborhood, a heavily Jewish area, was holding a Shabbat religious service at the time of the shooting.
Police are normally only present at the synagogue for security on high holidays, Michael Eisenberg, former president of the synagogue, told KDKA.
"On a day like today, the door is open, it’s a religious service, you can walk in and out," he said.
Around the time, three congregations amounting to about 100 people would have been using the building, Eisenberg said.
Shortly after reports of the shooting emerged, US President Donald Trump said in a tweet he was watching what he described as a "devastating" situation.
Trump told reporters later that the killings might have been prevented if there had been an armed guard in the building.
“If they had some kind of a protection inside the temple maybe it could have been a much more different situation, they didn’t," he said when asked about a possible link to US gun laws.
Jonathan Greenblatt, CEO of the Anti-Defamation League, said, "This is likely the deadliest attack on the Jewish community in the history of the United States."
On April 13, 2014, a pair of shootings occurred at a Jewish Community Center and a Jewish retirement community, both located in Overland Park, Kansas. A total of three people were killed in the shootings.
In 2015, a white supremacist murdered nine African Americans during a prayer service in Charleston, South Carolina.
In 2012, a neo-Nazi gunman with white supremacist ties walked into a Sikh gurdwara - or house of worship - in Oak Creek, Wisconsin, and murdered six Sikh Americans.