My recollection is that it came out over the next few months that there had been incidents of burn through of one or more O-rings, but that it hadn’t made it through all three.
The eruption of exhaust gas through all three O-rings caused an immediate and measurable decrease in SRB internal pressure. This was seen in the Challenger telemetry data. It also caused a visible plume to be emitted from the SRB body; if that had happened before the Challenger explosion, I’m sure it would have been made public by now. Space shuttle launches were well covered by long range cameras all the way to the point of SRB separation.
Please see my post #32.