The shooter - identified as a 46-year-old local resident named Robert Bowers - reportedly yelled "All Jews must die" as he burst into the Tree of Life synagogue, where dozens of congregants were gathered in the morning for Sabbath services.
An assault rifle and three handguns were found on the suspect, Bob Jones, FBI Pittsburgh special agent in charge, said during a press conference Saturday afternoon.
Pittsburgh police chief Scott Schubert said that he arrived at the scene shortly after the shooting.
Three officers were shot in the Saturday morning attack at the Tree of Life Congregation in the city's Squirrel Hill neighborhood, and a local hospital said it was treating multiple victims.
Bowers has an active license and has made at least six known firearm purchases since 1996, a law enforcement official familiar with the investigation said. Investigators said a post on Gab believed to be by Bowers said, "can't sit by and watch my people get slaughtered".
"When you go into a synagogue, saying, 'I want to kill all the Jews, ' that's a hate crime".
Social media posts by someone with the name Robert Bowers were also reported to be full of anti-Semitic comments.
Visibly moved Wendell Hissrich, the Pittsburgh public safety director."It is a very horrific crime scene".
"In the last five years, we've seen a precipitous rise in anti-Semitism", Nasatir said.
HIAS is a Maryland-based non-profit group that helps refugees around the world find safety and freedom.
The neighbourhood where the attack occurred, Squirrel Hill, has a vibrant Jewish community and is home to several synagogues, kosher restaurants and bakeries.
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The Jerusalem Post quoted Dov Khenin, a member of the Joint List Arab faction in the Knesset, as saying that he suspects "the attack is an expression of the growth of risky antisemitism in American society - a part of the racist and violent wave during the presidency of [Donald] Trump".
"There is no place in our nation for senseless violence and hatred, We must remember, there is more that brings us together than divides us", the council said in an emailed statement.
Shabbat services at Tree of Life are usually led by Rabbi Hazzan Jeffrey Myers at 9:45 a.m. on Saturday, according to the synagogue's website.
He was getting ready for services when he received a phone call from a member who works with Pittsburgh's Emergency Services, saying he had been notified through scanner and other communications that there was an active shooter at their synagogue.
"You wouldn't think this would be possible in this day and age but we just don't seem to learn from the past", he said.
He says the general mood is one of shock as details continue to be released about victims.
Reports indicate that at least four people were killed.
"We will always stand with the Jewish community against hate and violence", he said.
Police told the Pittsburgh Post-Gazzette Bowers was not known to police other than "contacts" with him more than a decade ago but the nature of those incidents were not revealed to reporters.
About 2,000 people came to the vigils, which were held at the Sixth Presbyterian Church and also at Murray and Forbes avenues about a half mile from the synagogue, an event organized by area high school students. "It's a time for people to really reach out".