At least four people have been killed in Saturday morning's shooting at the Tree of Life synagogue in Pittsburgh, according to a city official.
The shooter surrendered to Pittsburgh police and was being transported to Mercy Hospital, said Curt Conrad, chief of staff for City Councilman Corey O'Connor.
Another law enforcement official told CNN at least 12 people have been shot.
Three police officers were shot, officials said earlier at an impromptu press conference. It's unclear if they are part of those casualties.
"There are fatalities," said Allegheny County Executive Rich Fitzgerald.
Pittsburgh police Cmdr. Jason Lando previously said there were "multiple casualties." Officers were dispatched to the scene after receiving reports of active gunfire at the synagogue, he said.
"It is imperative that the neighbors in the community surrounding the Tree of Life synagogue stay in their houses and shelter in place," Lando said. "Do not come out of your home right now. It is not safe."
Fred Rabner, a member of the synagogue, said it was a "close-knit community," and that everyone was calling around to make sure their loved ones are OK.
"Everyone is just shaken up and upset," Rabner said. "It's awful, it's just awful."
The Tree of Life synagogue is a conservative Jewish congregation, according to its website. It's in Squirrel Hill, a historic, Jewish neighborhood.
The synagogue has a Shabbat service at 9:45 a.m. Saturdays, the website said.
President Donald Trump tweeted he was aware of the situation in Pittsburgh and that law enforcement was on the scene.
"People in Squirrel Hill area should remain sheltered," Trump said. "Looks like multiple fatalities. Beware of active shooter. God Bless All!"
Pennsylvania Gov. Tom Wolf said on Twitter that it was a "serious situation," and the state police were helping local first responders.
Carnegie Mellon University issued a shelter in place advisory for its campus, which is near the synagogue.
CNN's Joe Sterling contributed to this report.