At least 10 people have been killed after a gunman opened fire during a baby naming ceremony inside a synagogue in Pittsburgh.
Public safety director Wendell Hissrich described the crime scene as "horrific", saying it was one of the worst he had ever seen.
At least six other people were wounded, including four police officers who rushed to the scene.
Police said a suspect, who reportedly yelled antisemitic remarks, was in custody after the attack at the Tree of Life Congregation in the Squirrel Hill neighbourhood.
A police official identified him as Robert Bowers and said he is in his 40s, adding that they are treating the attack as a hate crime.
Donald Trump addressed the incident on Twitter almost immediately, saying he was watching events unfold and people should remain sheltered.
A few hours later on his way to address a Future Farmers of America Convention, the president defended gun laws as he answered questions from journalists.
Mr Trump called the shooting "far more devastating than anyone thought," saying: "It's a terrible thing what's going on with hate in our country."
But he went on to say the outcome might have been different if the synagogue "had some kind of protection".
He suggested an armed guard might be a good idea for all churches and synagogues.
Israeli Prime Minister Benjamin Netanyahu said he was "heartbroken and appalled" by the attack.
"The entire people of Israel grieve with the families of the dead," he said.
"We stand together with the Jewish community of Pittsburgh. We stand together with the American people in the face of this horrendous antisemitic brutality. And we all pray for the speedy recovery of the wounded."
World Jewish Congress President Ronald S Lauder called the shooting "an attack not just on the Jewish community, but on America as a whole".
The synagogue is a fortress-like concrete building, its facade punctuated by rows of swirling, modernistic stained-glass windows illustrating the story of creation, the acceptance of God's law, the "life cycle" and "how human-beings should care for the earth and one another", according to its website.
At the farmers convention, Mr Trump began by addressing the shooting.
"As you know, earlier today there was a horrific shooting targeting and killing Jewish Americans," he said.
"This wicked act of mass murder is pure evil, hard to believe and frankly something that is unimaginable.
"Our nation and the world are shocked and stunned by the grief.
"This was an antisemitic act.
"You wouldn't think this is possible in this day and age.
"Our minds cannot comprehended the cruel hate and the twisted malice that could cause a person to unleash such terrible malice during a baby naming ceremony."
He called antisemitism "vile and hate-filled" and said there must be no tolerance for any form of religious hatred.
"We are praying for the families of the victims," he added.
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He praised the officers involved, and said the ones that were hurt have been "very badly wounded".
"We mourn for the unthinkable loss of life today."