In the videos of the Columbia descent, the trail show a bulging plume just before the break up.
Could one of the dehumidifiers related to the research package, explode? There was a report of a malfunctioning and over heating.
Maybe one of the landing carriage tires exploded? These would be liquid filled ?
The fuel tank insulation debris hitting the left wing of Columbia after liftoff, could have been the cause. The Ceramic Heat Tiles /a> were very brittle.
regards,