The Allegheny County Police Department, which is investigating the encounter, said that two firearms were found on the floor of the car. When asked if the teenager was found with a weapon on his person, Coleman McDonough, the department’s superintendent, said he was not.

The Pittsburgh Post-Gazette on Wednesday quoted Mayor Louis Payne of East Pittsburgh as saying that the officer who shot Antwon was hired in mid-May and had been formally sworn in hours before the shooting.

In a statement on Wednesday night, S. Lee Merritt, a lawyer representing Antwon’s family, said: “We know very little about the circumstances surrounding his death at this early stage. We must emphasize that rumors of him being involved in a separate shooting are unsubstantiated. We know that he was not armed at the time he was shot down, that he posed no immediate threat to anyone, and that, significantly, the driver of the vehicle he occupied was released from police custody.”

On Wednesday evening, dozens of people gathered outside the East Pittsburgh Police Department to protest the black teenager’s death. “No justice, no peace!” they chanted. Some carried signs that said, “Justice4Antwon” and “#BlackLivesMatter.”

Those who knew Antwon described him as bright, lively and funny. He was a senior at Woodland Hills High School who was expected to graduate at the end of the year, the superintendent, Al Johnson, said in an interview on Wednesday.