Pennsylvania officer shot, killed 17-year-old boy who was fleeing at traffic stop

Another person who fled the scene is being sought.

Police shot and killed an unarmed 17-year-old boy who fled from them near Pittsburgh Tuesday after he was pulled over in a car believed to have been connected to an earlier shooting that night, authorities said.

The initial shooting was reported in North Braddock at 8:27 p.m., the Allegheny County Police Department said Wednesday.

Responding officers found a 22-year-old man wounded, struck in the abdomen, police said.

The shooter had fired nine rounds at the man from a passing car, police said. The victim also returned fire at the passing car, police said.

The man was taken to a hospital and later released, police said.

"Witnesses described the vehicle involved in the shooting, and the description was broadcast" to officers in the area, the Allegheny County police said in a news release Wednesday.

An East Pittsburgh police officer spotted a car matching the description -- a silver Chevy Cruze -- and pulled the car over at 8:40 p.m., police said.

The driver, 20, was ordered out of the car and directed to the ground, police said.

But two other people who were in that car fled on foot, police said.

The officer shot three times at one of the fleeing people -– the 17-year-old -- and struck him three times, police said.

The 17-year-old, Antwon Rose, was taken to a hospital where he died, the medical examiner's office said.

Rose did not have a weapon on him and none of the three suspects fired at officers, police said, adding that "two firearms were later recovered from the suspect vehicle."

The driver was detained and later released, police said.

But the other person who fled the silver Chevy Cruze remains at large, police said. That person's identity is unknown.

The East Pittsburgh officer involved in the shooting, who has not been publicly identified, is on administrative leave, police said.

The East Pittsburgh officer had neither an in-car camera nor a body camera, police said.

Findings of the investigation will be presented to the district attorney's office, police said.

ABC News' Fergal Gallagher and Devin Villacis contributed to this report.