Police are viewing the blast that shook San Francisco's Sunnyside neighborhood Monday night as an accident. Although illegal fireworks were found in the house's garage and rumors swirled about possible mischief, police still aren't sure what caused the explosion. The arson team at the scene ruled out any criminal activity. The blast - which blew off all but the top two steps on the staircase leading up to the second floor, as well as much of the lower front of the building - sent two residents to the hospital. Both are listed in critical condition.