Immediately after the fire, NASA convened the Apollo 204 Accident Review Board to determine the cause of the fire, and both houses of the United States Congress conducted their own committee inquiries to oversee NASA's investigation. The ignition source of the fire was determined to be electrical, and the fire spread rapidly due to combustible nylon material, and the high pressure, pure oxygen cabin atmosphere. Rescue was prevented by the plug door hatch, which could not be opened against the internal pressure of the cabin. Because the rocket was unfueled, the test had not been considered hazardous, and emergency preparedness for it was poor.