No point speculating at this stage since we don't have any evidence, but if I had to make a guess I'd go with right wing WASP nutjob. Statistically, there are more of them in the US than any of the other options.
Yes, I thought so too, some christian extremist - they been surprisingly quiet on the terrorist front as of recent times and it seems that Muslims always seem to dominate the headlines in this field. Even if its not them, at some time or other retalliation is bound to happen, and pretty much what was happening in the middle ages.
Not only does religion slow progress, there a danger it could reverse it.