A 16-year-old mentally challenged boy stole a SEPTA bus in Upper Darby Wednesday morning and crashed the bus into 20 cars, police said.

The teenager told police that he wanted to practice driving.

The boy stole a SEPTA bus from Victory Depot at 69th Street and Victory Avenue in Upper Darby a little before noon, drove down West Chester Pike, made a drastic U-turn and barrelled down 20 parked cars in a parking lot. One of the cars had four people in it. None were injured.

"The other cars and poles slowed him down, but he was coming really fast," witness Tranesha Mosley told NBC Philadelphia. Mosley was sitting on her SUV with three other people when the bus came crashing through the parking lot.

"I was just praying," said Mosley. "I had my 2-year-old on my lap."

The accident left a utility pole dangerously dangling over the lot, and knocked out the power lines.

The teen said that he found the bus running with the keys in the ignition, according to police.

SEPTA spokesman Richard Maloney said that the keys to the bus should not have been in the ignition, but turned in at the depot. SEPTA is investigating how the boy accessed the keys and the bus.