SO FAR ALL THE COLUMBIA INVESTIGATION SEEMS TO AGREE WITH THIS initial assessment on InstaPundit:

From the video it looks like structural failure, followed by an explosion as the spacecraft disintegrated. That’s unlikely to be the result of sabotage. Most likely it was failure in a wing spar or some other component, probably brought on by age and fatigue, though possibly caused by tile zippering and burn-through, or damage on launch.

It looks like zippering and burn-through, brought on by damage at launch made more severe by age and fatigue. Now this article in the Washington Post notes that the Shuttle fleet may just be too old and worn out to fly much more:

The shuttles — the only reusable manned spacecraft operating in the world — were difficult enough to maintain when their millions of parts were new. The effects of aging have added to the challenge, as well as adding an immeasurable degree of uncertainty.

The Columbia investigators have concluded that damage to the tough carbon composite material that shields the wings against the heat of reentry precipitated that disaster, and tests have also shown that a hidden effect of aging — oxidation that eats into surfaces — might have degraded the heat shielding and contributed to the deadly chain of events. But no one knows for sure.

NASA, unfortunately, is sufficiently anxious to keep the Shuttle fleet flying — and sufficiently underfunded — that it hasn’t looked very far into alternatives.

This is, of course, a reason why relying on a single fragile government program is silly.