It happened around 9:30 a.m. A truck was heading east on the Schuylkill Expressway, away from the city, when it lost a tire.
The tire jumped across the median barrier, hit the front of an inbound car, and then went airborne, flew across Vare Avenue and crashed through the roof of the PHA Workforce Development training center.
"We thought it was a plane tire. We said, 'Ain't no way in the world a tire could get this high,'" said PHA Training Supervisor George Johnson, Jr.
The tire punched a hole in one side of the roof and then landed on the other side. Workers in the building say it sounded like an explosion.
"We heard a heavy thump," said Johnson. "And my instructors came out and told me, 'Look, we smell gas.' So they put a ladder on the roof to see what was that thump that hit the roof. And when they put the ladder on the roof they saw this giant tire on the roof. And then we climbed to the peak and saw a big gash in the roof."
They quickly cleared everyone out because of the gas odor, which turned out to be caused by a plumber working on appliances.
Video shows the damage to Honda Accord that got hit. 33-year-old Kiwon Lee was at the wheel, on the way to his job at Childrens Hospital. His pregnant wife, 26-year-old Ahyeon Baek, was in the passenger seat.
Lee says it seemed like the tire fell from the sky. They were jolted, but uninjured.
As a precaution, Ahyeon, who is 7 months pregnant, was taken to the hospital where she'll spend the night under observation. Lee says they're lucky to be alive.
Back at the PHA building, workforce trainees say they were getting ready for a test one minute and running from the building the next.
"We didn't know what it was. And a few minutes later the teachers came in and told us we had to evacuate the building and everyone had to go outside," said trainee Juwan Campbell. They were shocked to see the damage to their building and the damage to Kiwon Lee's car.
"We saw the car out here from the accident. But the tractor trailer that the wheel came from was nowhere around," said trainee Razeik Banks.
There were no injuries inside the building.
"There were no injuries, just damage to property, thank God," said Interim PHA Executive Director Kelvin Jeremiah.
Pennsylvania State Police are now looking for the truck that lost the tire.
They say the driver probably didn't even know he lost the tire until stopping somewhere down the road later on Monday.
Anyone who witnessed the crash or the tire coming off of a tractor trailer is asked to contacted PSP Philadelphia at 215-560-6200.