I keep seeing these different video clips of the breakup, and to me it looks increasingly like Columbia lost a wing or the vertical stabilizer. Something broad and flat appears to flip away just after the breakup begins.

Interestingly, the Arlington home-video clip shows the trail turning a deep brownish-red or purplish-red just before anything appears wrong with the Orbiter itself (well, unless the trail itself was a problem, but it looked normal as I recall up to that point). They hypergolics are described as brownish gases, so perhaps there was a leak from one of the tanks or a stuck-open thruster. I've no clue what color burning aluminum gives off, but an analysis of the trail colors in these videos could give hints of what was burning when. Unfortunately, I doubt the tapes are of such quality as to give more than a broad hint based on color.