BOSTON — A 17-year-old passenger panicked during a routine traffic stop in Roxbury, jumped out of the car holding a blue backpack, dropped it directly at an officer's feet, and took off running.

Inside the backpack: a loaded Sarsilmaz SAR-9 9mm semi-automatic pistol with 17 rounds in the magazine and a Solofish light and laser attachment.

The car had one driver and three passengers in the back seat. As officers approached, one of the rear passengers stepped out clutching the backpack, tossed it at the officer's feet, and fled on foot down Dale Street.

Officers chased him and caught him in the side yard of a Rockland Avenue residence after a brief foot pursuit. The remaining passengers were frisked and released. The driver was cited for the taillight and a rejected inspection sticker.

When officers examined the discarded backpack, they felt a hard metallic object through the exterior. Inside was the loaded pistol — a full-sized 9mm with a 17-round magazine and a laser sight bolted to the rail.

The juvenile was arrested and is expected to be arraigned in Boston Municipal Court's Juvenile Division on five charges:

A 17-year-old. A loaded semi-automatic with a laser. A Tuesday afternoon in Roxbury. And the only reason anyone knows about it is a busted taillight.