Erie Police Chief Rick Lorah said the 45-year-old male gunman was related to seven of the victims in Friday's shooting, while the eighth was a friend of the pregnant woman.
Video and audio evidence recovered by investigators showed two "distressed" women arriving at the home shortly before 5pm and pounding on the door, he said.
The gunman then came out and shot them before returning inside.
Police arrived to find the two women dead outside. Inside they found the bodies of a five-year-old boy, a one-year-old girl, two more women, two young men and the shooter.
Officers arrived within minutes of receiving an alert via ShotSpotter, a controversial gunshot detection system that relies on artificial intelligence and a network of microphones to identify gunshots.
All nine deaths occurred before officers arrived, the city said, and no additional shots were fired after that.
"Sounds heard during the police response that may have been mistaken for gunfire were flash bangs associated with law enforcement's tactical response," the city said.
A news conference was scheduled for Saturday afternoon.
"I have not seen anything like this in my 28-plus years," Lorah told a local news station.
"I'm not sure that the city of Erie has ever seen the loss of life and the devastation that we just witnessed."
Governor Josh Shapiro said he offered his administration's "full support" to Erie Mayor Daria Devlin.
Erie is about 190km north of Pittsburgh.
with Reuters
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