Query: What was the cause of the fire that killed everyone on the first Apollo mission before launch?
Context: On January 27, 1967, the same day the US and USSR signed the Outer Space Treaty, the crew of the first manned Apollo mission, Command Pilot Virgil "Gus" Grissom, Senior Pilot Edward H. White, and Pilot Roger Chaffee, were killed in a fire that swept through their spacecraft cabin during a ground test, less than a month before the planned February 21 launch. An investigative board determined the fire was probably caused by an electrical spark, and quickly grew out of control, fed by the spacecraft's pure oxygen atmosphere. Crew escape was made impossible by inability to open the plug door hatch cover against the greater-than-atmospheric internal pressure.