The Space Shuttle Columbia appears to have broken up in flight during its re-entry. More later.
Update: Although the CNN story I just cited mentioned terrorism concerns in the third graf, the network's TV coverage is quoting officials as saying the mishap is unlikely to be terrorism-related. Of course, it's much too early to draw many conclusions, aside from the obvious: the Columbia appears to have broken up during re-entry.
Update 2: CNN is showing launch footage that seems to depict a chunk of debris falling from the large, orange external fuel tank and striking the Columbia's left wing; however, they're also reporting that mission controllers at the time assessed the occurrence as posing no threat.
Update 3: As I feared, CNN is now reporting that, as the shuttle was traveling at more than Mach 6, there's no way the crew could have ejected. This news, while not at all unexpected, is sad indeed.