Police in Pittsburgh reported "multiple victims" after reporting on a synagogue report in a synagogue on Saturday morning, a police chief said.
Chris Togneri, a spokesman for the Pittsburgh Police Department, said three policemen had been shot and the suspect was in custody.
Commander Jason Lando told CNN, "We have several victims now, we are working on the situation." It was not immediately clear how many other people had been shot.
Erika Strassburger, councilor of the district where the synagogue is located, the Congregation for the Tree of Life, said it was not immediately clear how many people had been shot.
Strassburger said the gunman had been barricaded in the building before.
President Trump said on Twitter that he is watching the situation. He wrote: "Law enforcement on the ground. People in the Squirrel Hill area should stay protected. Looks like several fatalities. Beware of the active shooter. God bless everyone! "
Gov. Tom Wolf of Pennsyvlania said on Twitter that it was "a serious situation" and the state police support the local law enforcement agencies.
55-year-old Ben Opie, who sees the synagogue from his backyard, said his wife left home Saturday morning to volunteer when SWAT officers approached her home and said it was an active shooter in the synagogue.
"They chased my wife in," he said. "They only said that you should go into the house."
Justin Matase returned home at 10:30 am when he saw more than 30 police cars in the streets of Shady and Wilkins, he said. Mr. Matase said he lived less than 300 meters from the synagogue.
Mr. Matase said the neighborhood was mostly Jewish, and although he was not part of the congregation, he had many friends.