Police say a suspect is in custody after a gunman opened fire at Pittsburgh synagogue on Saturday morning, leaving at least 11 dead and six injured.
Three officers were also shot in the attack and one injured by shrapnel at the Tree of Life Synagogue in Pittsburgh's Squirrel Hill neighbourhood.
Local news accounts say the gunman shouted “All Jews must die,” as he stormed the synagogue during a baby naming ceremony.
Law enforcement officials have identified the gunman as 46-year-old Robert Bowers.
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 can’t sit by and watch my people get slaughtered. Screw your optics, I’m going in.”
The comment was posted on Gab, a social networking service created as an alternative to Twitter and is based in Philadelphia, Pennsylvania. 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 tree-lined residential neighbourhood of Squirrel Hill, about 10 minutes from downtown Pittsburgh, is the hub of the city's Jewish community.
“It’s a horrific crime scene,” said Pittsburgh Public Safety Director Wendell Hissrich. “It’s one of the worst I’ve ever seen.”
He added that the shooting is being considered as a hate crime, so will be under federal investigation.
Attorney General Jeff Sessions confirmed Mr Hissrich's comments, saying the Justice Department intends to file hate crime and other charges against the man accused.
In a statement, Mr Sessions said the killings were "reprehensible and utterly repugnant to the values of this nation."
US President Donald Trump said he had been monitoring the shooting. In a tweet, he encouraged people to shelter in place and said "looks like multiple fatalities."
Mr Trump later told reporters that the shooting had been "far more devastating than anybody originally thought."
"It's a terrible, terrible thing what's going on with hate in our country, frankly, and all over the world," he said before leaving for a series of campaign events in Indiana and Illinois.
"Something has to be done," the president said. "When people do this, they should get the death penalty."
He said the suspect was “a mad man, a whacko,” yet when challenged on the nation’s current gun laws, he suggested that the toll might have been far lower had armed guards been posted in the synagogue, which they reportedly do only on the religion's High Holy Days.
"If they had protection inside the temple, maybe it could have been a very much different situation," he said. "They didn't."
But Trump did call for stiffer penalties and speedier justice.
"I think they should very much bring the death penalty into vogue," he said. "Anybody who does a thing like this to innocent people in temple, in church... they should really suffer the ultimate price."
Pittsburgh's sports teams, the Pittsburgh Steelers and Pittsburgh Penguins, are expressing their condolences for the deadly shooting.
On their Twitter pages, both teams said their "thoughts and prayers" are with all those affected by the shooting.