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. The board also found design and construction flaws in the spacecraft, and procedural failings, including failure to appreciate the hazard of the pure-oxygen atmosphere, as well as inadequate safety procedures. All these flaws had to be corrected over the next twenty-two months until the first piloted flight could be made. Mercury and Gemini veteran Grissom had been a favored choice of Deke Slayton, NASA's Director of Flight Crew Operations, to make the first piloted landing.