Eleven people have been killed, officials say, in a gun attack on a synagogue in the city of Pittsburgh, Pennsylvania.
The gunman, who opened fire as the Tree of Life synagogue held a service, was later taken into custody.
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.
How did the shooting unfold?
Worshippers had gathered at the synagogue, in the Squirrel Hill neighbourhood, to mark 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, a heavy-set white male armed with an assault rifle and two pistols, entered the building during the Saturday morning service.
Reports say the gunman barricaded himself in a room at the synagogue.
Emergency services arrived at the building at about 10am local time and gunshots could be heard.
Pittsburgh's Public Safety Director Wendell Hissrich later confirmed the gunman 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 he did not know if Mr Bowers was 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.
President Trump's reaction
He called the shooting a "terrible, terrible thing".
"To see this happen again and again, for so many years, it's just a shame," he told reporters.
He described the gunman as a "maniac" and suggested the US should "stiffen up our laws of the death penalty".
"These people should pay the ultimate price. This has to stop," he said.
Mr Trump added that the incident had "little to do" with US gun laws. "If they had protection inside, maybe it could have been a different situation," he said.
The president later appeared at the Future Farmers of America Convention in Indianapolis, saying: "There must be no tolerance for anti-Semitism. It must be condemned and confronted everywhere and anywhere it appears."
Pennsylvania Governor Tom Wolf said in a statement that the incident was an "absolute tragedy" and that such acts of violence could not be accepted as "normal".
He said: "These senseless acts of violence are not who we are as Americans.
"My thoughts right now are focused on the victims, their families and making sure law enforcement has every resource they need."
The Jewish Federation of Greater Pittsburgh's president, Jeff Finkelstein, said his "heart goes out to all these families".
"Now I'm just sad. This should not be happening. Period. It should not be happening in a synagogue. It should not be happening in our neighbourhood here in Squirrel Hill," he said.
A vigil will take place in Squirrel Hill later today.
Israeli Prime Minister Benjamin Netanyahu denounced the shooting as "horrendous anti-Semitic brutality".
The New York Police Department said it had deployed officers to synagogues throughout the city as a precaution.
The BBC's Dan Johnson in Washington said the shootings came at a tense time in the US, after a week in which mail bombs were sent to critics of Mr Trump, ahead of crucial mid-term elections next month.