Investigators are working to uncover what caused a fleet of decommissioned SEPTA buses to catch fire at a Philadelphia bus depot on Thursday morning. SEPTA GM Scott Sauer said over a dozen of the torched buses were Protera-manufactured electric buses that he said have not been used in several years. A total of 40 buses were damaged in the fire, sparking air quality concerns that the city's health department says are still unhealthy for nearby residents. No one was injured in the fire at SEPTA's largest bus depot that runs 24-hours a day.