I remember Joan Rivers was subbing for Carson that night and she came out, dressed all in black, and said simply, “It’s been a bad day.”.......................
One early theory was that there was a “water hammer effect” in the 18” LOX duct that carried oxidizer from the nose of the the external tank to the engines.
I recorded all the TV coverage I could find on my (brand new) VHS VCR, and frame-stepped through the explosion footage. There was one particular frame in which the LOX tank ruptured and exploded, it looked like a gigantic blue-white electric arc.
Before the truth came out, the “water hammer effect” was about the only explanation going. IIRC, it took maybe a week before the “SRB field joint” explanation began to be heard.