They would have sensor relay from tail section...multiple.
They would have last cycling info on computer systems...heat info...termination of signal values on sensors.
.... my gut instinct says catastrophic event which terminates electrical feed.
A computer cascade event?
Columbia sent into a angle which termiantes com transmission ability in ionization.....inverted?...sideways[Shearing]?...tumbling?
Columbias computer suite is said to have redundant back up to adjust in descent if profile angle is off...they have time to correct.
Did they?
Where they trying..except ionization prevent com link with Houston.
Unless a cascade event occure with computer..even if Columbia was out of descent profile..you still have cockpit awareness...telemetry with Houston..and com link.
Com link drops out..yet Columbias profile is nominal as per Nasa's breifing to us.
Something unrecoverable occured.