A gunman stormed a Pittsburgh synagogue during Saturday services, killing 11 worshippers and wounding six others, including four police officers, before he was arrested.
US President Donald Trump said he would travel to Pittsburgh, Pennsylvania, after the shooting, although he did not say when he would visit.
"It's a very horrific crime scene, one of the worst that I've seen," Pittsburgh public safety director Wendell Hissrich told a news conference near the scene.
"This falls under hate crime," he said, adding there was no active threat to the community and that the shooter had been taken to a hospital.
KDKA television cited police sources as saying the gunman walked into the building and yelled "All Jews must die".
Three police officers were shot and one was injured by shrapnel, Alleghany County spokeswoman Amie Downs said in an email.
Two of the six people injured were in critical condition, Ms Downs said, but would not immediately say if the count of six injured people included the suspect.
Authorities and elected officials were expected to provide more details at a news conference. The Federal Bureau of Investigation will lead the probe into the attack.
US attorney for western Pennsylvania Scott Brady told a news conference that charges could be filed later 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," Mr 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 organisation believed Bowers was acting alone, adding: "We have no knowledge that he was known to law enforcement before today."
A social media post by Bowers said a Jewish refugee organisation, the Hebrew Immigrant Aid Society, "likes to bring invaders in that kill our people. I cant sit by and watch my people get slaughtered. Screw your optics, Im 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.
"Gab took swift and proactive action to contact law enforcement immediately," it said.
"We first backed up all user data from the account and then proceeded to suspend the account. We then contacted the FBI and made them aware of this account and the user data in our possession."
The shooting, for which one federal law enforcement official said Bowers used an AR-15 semi-automatic rifle, 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, its 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, Mr Eisenberg said.
Most of the congregants were older people, according to a former rabbi interviewed by local media.
As you know, earlier today there was a horrific shooting targeting and killing Jewish Americans at the Tree of Life Synagogue in Pittsburgh, Pennsylvania. The shooter is in custody, and federal authorities have been dispatched to support state and local police... pic.twitter.com/WqO7GfPyMT
— Donald J. Trump (@realDonaldTrump) October 27, 2018
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.
Mr 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 didnt," he said when asked about a possible link to US gunlaws.
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 13 April 2014, a pair of shootings occurred at a Jewish community centre and a Jewish retirement community, both located in Overland Park, Kansas. A total of three people were killed in the shootings.
Israeli Prime Minister Benjamin Netanyahu said he wash eartbroken, describing the attack as "horrendous anti-Semitic brutality."
