At least eleven people have been shot dead at a Pittsburgh synagogue.
A gunman has opened fire during a baby-naming ceremony at a Pittsburgh synagogue and people with knowledge of the investigation say at least 11 people have been killed.
At least six other people were wounded, including four police officers who dashed to the scene, authorities say.
Police say a suspect is in custody after the attack at the Tree of Life Congregation in Pittsburgh's Squirrel Hill neighbourhood on Saturday.
A law enforcement official identified the suspect as Robert Bowers, 46, whose online posts were reportedly rife with anti-Semitic comments.
City officials say the shooting is being investigated as a federal hate crime. It comes amid a rash of high-profile attacks in an increasingly divided country, including the series of pipe bombs mailed over the past week to prominent Democrats and former officials.
The shooting also immediately reignited the longstanding national debate about guns: President Donald Trump said synagogues and churches should have armed guards, while Pennsylvania's Democratic governor said that "dangerous weapons are putting our citizens in harm's way".
"It is a very horrific crime scene. It's one of the worst that I've seen and I've been on some plane crashes," said a visibly moved Wendell Hissrich, the Pittsburgh public safety director.
The attack took place during a baby-naming ceremony, according to Pennsylvania Attorney General Josh Shapiro. It was unknown whether the baby was harmed.
The synagogue is located in the tree-lined residential neighbourhood of Squirrel Hill, about 10 minutes from downtown Pittsburgh and the hub of Pittsburgh's Jewish community.
'Our hearts break'
Israeli Prime Minister Benjamin Netanyahu voiced his grief and solidarity with the United States, saying he was "heartbroken and appalled by the murderous attack."
"The entire people of Israel grieve with the families of the dead" Netanyahu said in a video message. "We stand together with the Jewish community of Pittsburgh. We stand together with the American people in the face of this horrendous antisemitic brutality."
"We are devastated," tweeted Jonathan Greenblatt, the head of the Anti-Defamation League.
"Jews targeted on Shabbat morning at synagogue, a holy place of worship, is unconscionable. Our hearts break for the victims, their families, and the entire Jewish community."
Trump's daughter Ivanka, in a tweet, vowed that "America is stronger than the acts of a depraved bigot and anti-Semite."
"All good Americans stand with the Jewish people to oppose acts of terror & share the horror, disgust & outrage over the massacre in Pittsburgh. We must unite against hatred & evil. God bless those affected."
The New York Police Department said it was deploying heavy weapons teams to houses of worship across the city in response to the Pittsburgh attack.
It is the latest shooting incident in the United States, where gunmen regularly cause mass casualties and firearms are linked to more than 30,000 deaths annually.
The Tree of Life Synagogue is in a residential neighborhood about five miles (eight kilometers) east of downtown Pittsburgh.
Michael Eisenberg, past president of the synagogue, told local television the door would typically have been open on Saturdays with religious services going on. He said police are normally deployed only on High Holy Days - the holiest annual Jewish religious holidays.
He said security was a "major concern" during his stint as president, and active shooting situations and active shooter trainings were conducted, "if something horrific like this happened.
Anti-Semitism and hate crimes have been on a rise in the United States in recent years - with anti-Semitic incidents surging 57 percent from 2016 to 2017, to 1,986 from 1,267, according to the ADL.
Squirrel Hill has historically been the center of Jewish life in the greater Pittsburgh and is home to 26 percent of all Pittsburgh-area Jewish households, according to a study from Brandeis University.
More than 80 percent of the neighborhood's residents said they had some concern or were very concerned about rising anti-Semitism, found the 2017 study.
The Tree of Life congregation was founded more than 150 years ago and in 2010 merged with the five-year-old Or L'Simcha congregation.
Additional reporting by AFP.