EAST PITTSBURGH, Pa. — A 17-year-old boy fatally shot by a police officer in Pennsylvania seconds after he fled a traffic stop did not pose a threat to anyone, a lawyer for the family of the teen said.

Civil rights attorney S. Lee Merritt said late Wednesday that he doesn't see any apparent justification for the use of deadly force by an East Pittsburgh police officer that left Antwon Rose Jr. dead. Allegheny County police are conducting an independent investigation of the shooting in East Pittsburgh, a borough about 10 miles (16 kilometers) east of Pittsburgh.

Part of the encounter was captured on video and posted to Facebook by a bystander. Officer Michael Rosfeld confirmed to a WTAE-TV reporter who went to his house Thursday that he was the officer who fired the shots but said he had not watched the news and was unaware the video even existed. Rosfeld said he could not talk about the shooting because of the open investigation.

Investigators and city officials have declined to name the officer. They said he was placed on administrative leave during the investigation.

The shooting has sparked some social media outrage and calls for punishment of the officer, including from rapper Nas and a handful of other celebrities. A Wednesday night protest at the East Pittsburgh police headquarters lasted for several hours and drew more than 100 people, some of whom laid down in front of a police cruiser.

A second protest Thursday afternoon at the Allegheny County courthouse attracted nearly 1,000 people, including speakers decrying police use of force and gun violence. Protesters later took to the streets, shutting down a major interstate. Some sat down on Interstate 376, blocking traffic in both directions, while others chanted, "What do we want? Justice. When do we want it? Now."

Investigators said the officer stopped the car Antwon and two other people were riding in Tuesday because it matched the description of a car reported to be involved in a shooting about 15 minutes earlier in a nearby town. As the officer took the driver into custody, the short video shows Antwon and the other passenger running from the car.

The officer quickly fired three shots, all of which struck Antwon, who later died at a hospital from his injuries. The medical examiner has not said where the teen was struck.

"We must emphasize that rumors of (Antwon) being involved in a separate shooting are unsubstantiated," Merritt wrote in his statement, saying the use of deadly force seems unjustified. "We know that he was not armed at the time he was shot down, that he posed no immediate threat to anyone."

Antwon's friends, family and teachers said he was a promising student, who volunteered at a charity, was generous and had a "million-dollar smile."

Allegheny County Police Commissioner Coleman McDonough said Wednesday that he is confident the car Antwon was in was involved in the earlier shooting, partly because a window had been shot. He said officers found two guns in the car, and that the driver was released after questioning without being charged.

McDonough confirmed that Antwon was not armed and that no shots were fired at the officers.

Pennsylvania law allows officers to use deadly force against a fleeing suspect in only a handful of circumstances. It's permitted if the suspect poses a threat of immediate danger, has used or threatened lethal violence previously or possesses a lethal weapon.

East Pittsburgh Mayor Louis Payne said the officer had worked for the department for two weeks and was officially sworn in a few hours before the shooting. Rosfeld confirmed to WTAE-TV that he has worked with the department almost three weeks and worked previously in the Oakmont, Harmar and University of Pittsburgh police departments.

Leaders of the Pittsburgh-based Black Political Empowerment Project asked Thursday for the Pennsylvania attorney general's office to investigate this and other police-involved shootings, citing only two instances where the Allegheny County district attorney has filed homicide charges against police officers in fatal shootings in the last 20 years.

A joint statement from East Pittsburgh officials and police issued Thursday expressed condolences and sadness over Antwon's death and asked the community to respect the investigative process. Rossfled said he has a lot of support from the local law enforcement community and from family.

Democratic U.S. Sen. Bob Casey also issued a statement Thursday saying the teen's family has a right to answers.

"I am disturbed by what I saw on the video, and I have numerous questions about exactly what happened and why," he said.