IIRC, it was an “o” ring.
My dad worked for Morton Thiokol at the time.
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.
That is the official story. However, on the morning of the launch there was a crack in the support structure connecting the shuttle to the fuel tank. Vibration during flight broke the structure and punctured the fuel tank under the rear of the shuttle. There is evidence for this in Challenger videos. Just before the explosion there is fire just under the rear of the shuttle next to the fuel tank. Secondly watch the video of the boosters after the explosion. Both are flying in an identical fashion with exhaust coming from the rear nozzle indicating the solid fuel is still burning. There is no indication of a burn through to damage the fuel tank on either booster which would still have been spewing smoke from the O-ring burn through.
The video of Ronald Reagans speech is clear on these issues. A contractor took the hit and NASA was protected.