Eleven people have been killed, officials say, in a gun attack on a synagogue in the US city of Pittsburgh, Pennsylvania.
The gunman, who opened fire as the Tree of Life synagogue held a service, was later taken into custody.
President Donald Trump said “a lot of people” had been killed and injured in a “wicked act of mass murder”.
The suspect, who has been officially identified as Robert Bowers, 46, was injured and is receiving treatment.
Two other people are in hospital in a critical condition, police said.
Federal investigators are treating the shooting as a hate crime.
The Anti-Defamation League, a Jewish non-governmental organisation that fights anti-Semitism, said: “We believe this is the deadliest attack on the Jewish community in the history of the United States.”
Hundreds of people – from the neighbourhood and also all across Pittsburgh – later gathered for an interfaith vigil for the victims of the attack in Squirrel Hill.
Sophia Levin, a local resident and one of the organisers, told the BBC some people were “really sad” and the mood was “heavy”.
But she added that people also wanted to be “together, not alone”, and the vigil would help heal the city.
President Trump said he would visit Pittsburgh, following the attack.
On Saturday morning, worshippers had gathered at the synagogue, in the Squirrel Hill neighbourhood, for a baby naming ceremony during the Sabbath.
Squirrel Hill has one of the largest Jewish populations in Pennsylvania and this would have been the synagogue’s busiest day of the week.
According to reports Mr Bowers, a white male, entered the building armed with an assault rifle and two pistols.
Reports say he barricaded himself in a room at the synagogue when police approached.
Emergency services arrived at the building at about 10:00 local time (14:00 GMT), and gunshots could be heard.
Pittsburgh’s Public Safety Director Wendell Hissrich later confirmed Mr Bowers was in police custody and was being treated in hospital.
The crime scene was “horrific”, he told reporters. “One of the worst I’ve seen, and I’ve [worked] on some plane crashes. It’s very bad,” he added.
He said that two officers were injured in an “initial confrontation” and that a further two Swat officers were later hurt by the gunman when they entered the building. He said that no children were among the casualties.
US media said he had shouted “All Jews must die” as he carried out the attack.
Social media posts by someone with the name Robert Bowers were also reported to be full of anti-Semitic comments.
Pittsburgh FBI’s special agent in charge of the investigation, Bob Jones, told a press conference that Mr Bowers did not appear to be known to authorities prior to events on Saturday.
He said that any motive remains unknown but that authorities believe he was acting alone.
Mr Jones added that the investigation was “in the early stages”. “We will look at every aspect of the suspect’s life,” he said.
Mr Bowers is receiving treatment for what has been described as multiple gunshot wounds.