Or, failing that, to hit the external tank after launch, with the shuttle rolling and pitching into its climb attitude. The licks of flame at the base of the SRB occurred at most 2 sec before the main explosion. It was going at 2900 fps, so at best its altitude would have been 1 nautical mile lower when the bullet hit, meaning 8 nm altitude. Pretty far out to imagine a rifle bullet hitting at that point. There has been no public mention of the possibility of terrorism.
Terrorists claim credit for events.
To my knowledge, no one has claimed credit. There is a point in the redundancy argument that has bothered me since I interviewed at Stratus a year or so ago. Using the Stratus example, they run two copies of what they call a dipole. Each dipole is two mirror image processors with a high-speed comparator in the middle. When the live module gets a miscompare, it lights a LED and hands control over to the backup module.