The loss of the shuttle Challenger was not the United State's first loss of astronauts. Just 19 years earlier on January 27, 1967 the fledgling Apollo program came to an abrupt halt when a fire broke out during a routine test of the Apollo 1 capsule killing astronauts Gus Grissom, Ed White, and Roger Chaffee.
Although the exact cause was never found, it was believed to have been a short circuit that ignited combustible material, which in turn burned out of control in the 100% pure oxygen atmosphere of the Block I capsule.
The air pressure in the capsule was at 16 psi; normally in space the atmosphere would have been at 5 psi. The higher pressure helped the fire spread where it would have been more manageable otherwise.
As if that hadn't been enough, the crew was unable to escape the inferno because of the design of the capsule's hatch. It featured a double hatch assembly, including one that sealed tightly under pressure because it was an inward opening hatch. On top of that it required the removal of seven bolts before it could be opened inside the spacecraft.