Nelson said the shooting happened after several BART officers went to an apartment in a modern complex of three-story buildings to perform a search for a man suspected of committing a series of robberies on BART property.
According to law enforcement sources, the suspect was already in custody, having been arrested recently.
The officers - all of whom wore bulletproof vests, but were not part of a SWAT team - forced entry into the apartment, Nelson said, before "one officer fired a shot which fatally struck another officer.
Dog-faced officer got shot? Or maybe the officer was laying on the floor so his back was an enticing target.