ticket halls and tunnels wasn't easy. >> there was nothing quite like seeing a fire scene firsthand to give you a full appreciation of the devastation that has taken place, and also to give you a feel for how the fire might have developed. >> they needed to answer two critical questions -- what sparked the fire? and how could it appear so suddenly and erupt into a fatal blaze that claimed 31 lives? nearly a million people relied on the underground each day. they needed to know if the system was safe. >> it was on the front pages of every newspaper, obviously, for several days, and there was enormous demand to find out why it had happened. >> investigators started where the flames were first spotted, the escalator. this was the lowest point of the fire, indicating the fire had started there and burned upward. no accelerants or other indications of arson were found.