A pair of powerful storms that pounded Pittsburgh on Friday cut electricity to hospitals and universities and submerged several vehicles in a flash flood that killed three people.
Authorities told the Associated Press they were searching for other possible victims in the city's Highland Park neighbourhood, where muddy cars remained stranded on Washington Boulevard after the water had receded into the Allegheny River.
In at least one instance vehicles parked outside the WTAE studios were destroyed by power lines downed in the storms.
Rescue crews used inflatable boats to reach marooned drivers, some of whom said the water rose two meters and covered their vehicles.
Some drivers swam to safety, with Rhodearland Bailey, 79, being rescued from the roof of his car.
"I can swim a little bit and was looking for a tree branch," Bailey told the Pittsburgh Tribune-Review.
"I heard one woman yelling for help, but the water was coming down so fast, I couldn't see... I've never seen nothing like this in my life. Lord have mercy."
The flooding occurred in the late afternoon after the city was drenched with up to 76 millimeters of rain in an hour, the National Weather Service reported. The flash floods hit an area that experienced serious flooding last month.
Earlier on Friday, another storm caused power outages that led the University of Pittsburgh to close for the day. Parts of Carlow and Carnegie Mellon universities also lost electricity.