"Everything appeared normal"

There was no warning of a problem until the explosion, it seems. Columbia was NASA’s oldest shuttle but has been entirely refurbished somewhat recently. This means the cause of the accident is still a mystery. They also seem to be ruling out human error/pilot error.
A spokesman for NASA told people who may be in the vicinity of debris to avoid touching or moving it and to call the local authorities so that it may be cordoned off and impounded.
They are also asking people who may have taken pictures or shot videotape to turn over their footage to the appropriate authorities so that every scrap of evidence may be considered in the analysis of the causes of this disaster.