A man standing on a crowded Muni train pulls out a .45-caliber pistol.

He raises the gun, pointing it across the aisle, before tucking it back against his side. He draws it out several more times, once using the hand holding the gun to wipe his nose. Dozens of passengers stand and sit just feet away - but none reacts.

Their eyes, focused on smartphones and tablets, don't lift until the gunman fires a bullet into the back of a San Francisco State student getting off the train....