Disturbing body cam footage shows the moment a police officer fatally shot a 16-year-old boy fleeing from a gun-wielding youth at a California train station.

The San Diego PD on Friday released the surveillance video and body-worn camera footage from the station, capturing the entire January 28 incident as it unfolded at Santa Fe Depot in the city's downtown area.

Check out all the footage ... which begins inside the station when the teen is approached by two youths -- one brandishing a gun -- on a train platform.

The armed suspect aims the gun at the teen, who then runs off as shots are fired. It's unclear what triggered the shooting, but the teen was not hit by the bullets.

Police officers are already nearby responding to an unrelated incident when a call comes over about the shooting at the train station -- one cop dashes to the scene, arriving at the exact moment the fleeing teen emerges from the station directly in front of him.

With his gun drawn, the officer suddenly shoots the teen who collapses on the ground.

The cop, assisted by other officers, gives medical attention to the teen, finding a gun under his clothes while in the process. There's no indication the teen pulled out the weapon before he was gunned down. The unnamed teen was eventually rushed to a hospital, where he died.

Cops arrested the gun-toting youth who fired the shots that led to the fatal police encounter.

The San Diego District Attorney is reviewing the case to see if the shooting cop acted within the law.