[Gracey]

I'll assume that there were some critical tiles missing from liftoff, causing the spacecraft to burn up on reentry.

[Tex_GOP_Cruz]

>I'll assume that there were some critical tiles missing from liftoff, causing the spacecraft to burn up on reentry.

I would put that at the top of the list of possibilities. Hopefully enough debris will be found to make a determination.

[Ben Ficklin]

NASA's Mission Status Center is reporting witness reports mentioning parts disassociationg from the shuttle as it crossed California.

Mission Status Center