Even today, we still are not sure just what started the fire. Engineers checked every inch of Apollo 1, and found several places that may have started the fire. However, they could not tell which of them had. What they could tell, though, was that there were a lot of "flammable materials" – or things that would burn easily – in the spacecraft that were near each of the possible places where the fire started. The amount of flammable material, when mixed with the pure oxygen in the spacecraft, only needed a spark to start a serious fire.