The shooter was last seen traveling away from the scene in the Pontiac. Police released images of the vehicle Thursday.
Witnesses told KYW-TV they heard as many as 13 shots in quick succession Wednesday afternoon before seeing Semaj on the ground.
"I noticed that the young man had a wound to the neck, on the left side of his neck, so I instinctively just kicked in and grabbed someone's sweatshirt and correctly applied pressure to the area," medical assistant and witness Jahaira Cruz told KYW-TV. "As I was doing that, I had already started calling 911."
The news station reported police had a suspect involved in the shooting in custody, but authorities said the man was not suspected of shooting Semaj, but instead fired at the person who injured Semaj.
"He tries to cover the kid and protect the kid," Capt. John Walker told KYW-TV.
Police continue to investigate.