Answer
On January 27, 1967, a fire broke out during a pre-launch test for the Apollo 1 mission, killing the three astronauts aboard: Gus Grissom, Ed White, and Roger B. Chaffee. The fire was caused by an electrical short in the command module's oxygen tank, which ignited the spacecraft's atmosphere, which was made up of 100% pure oxygen. The fire spread quickly and the astronauts were unable to escape.