Over the coming weeks, a lot of engineers will be examining all the evidence from videos and radar returns to try and determine the breakup sequence.

One possible scenario getting attention is that damage to the tiles on the left wing caused a zipper effect, exposing the wing structure to the reentry heat. At the time of the breakup, the Columbia was at perhaps the most intense thermal portion of the reentry. This would've possibly caused the left wing to fail.

Just as speculation, had the left wing failed at that time, the Shuttle would probably have began to roll over, exposing portions of the vehicle that were not built to withstand the reentry heat. The cargo bay doors aren't built for that heat, nor are they built to withstand the likely aerodynamic loads. Odds are they would've failed. The upper wing surfaces aren't built to withstand the heat or aerodynamic loads, either. If the cargo bay doors came off, the interior structure would've been exposed to a lot of heat and aerodyamic loads, too, especially with the vehicle tumbling. They vehicle would've broke apart quickly. At Mach 18.3, there's a lot of heat and aerodynamic loads, even at 207,000 feet. Odds are the crew compartment was breached quickly. At least, I hope so.