An o-ring that was not suited for the cold weather that day, and the engineers new it.
NASA made big budgetary promises they could not keep, ignored the whistleblower, and killed 7 people.
And the really sad thing is the whole Space Shuttle program was a major waste of resources. We could of built something like 50 more Saturn V rockets and sent a lot more astronauts and payloads into orbit—for the billions we spent on the boondoggle of the Shuttle.
The re-usable aspect of the Shuttle was supposed to make it more economical....and it fit in well with the Boomer preoccupation with recycling, but that was all bunk. Used up a lot more money/time/resources, and in this case, lives....than if we’d relied on the tried and true rocket technology developed in Apollo.
The Russians are still using the same kinds of booster rockets they developed in the ‘60s...with rock solid reliability (and no ceasing of sending people up....).