IIRC, it was an “o” ring.
bfl
At the time, I was the Plans and Operations Officer for the 8th Infantry Division in Bad Kreuznach, Germany. I was called from my office by my guys in the Emergency Action Center. They had Armed Forces Network on the TV and they were getting the CNN feed. They were looping the explosion every few seconds. After a quick look, I walked down the hall to the Division Chief of Staff and told him what had happened and walked him back to the Emergency Action Center.
We had a contingency plan for any emergency related to the Space Shuttle, primarily for security in case the shuttle was forced to land in Spain or North Africa. The Chief of Staff was a bit of a nervous nelly in times like this so he ordered me to start alert for our contingency and to get the division in gear. I pointed out that I did not believe that any of the pieces would make it to Spain or Africa, but he was not having any of it. So the orders went out and I called the Corps and USAREUR EACs and told them they we were alerting and would be ready if needed. One of the comments was that this was serious and they didn’t need any joking around. We got a stand down message promptly.
My memory of the event is sort of macabre - there was at about the same time as the launch was scheduled also to be a major prize fight - I don’t remember between whom - but as I walked into the house at noon right after the disaster, my wife announced to me that “the Challenger blew up” and it took me a minute to grasp that she was talking about the space flight and not one of the fighters in the upcoming ring battle - just to add a little more bizarreness to the picture....
Imagine knowing and you did nothing about it
Understatement of the century?