Not lightening, global warming?
I don't mean to minimize the risk, other than the risk of being caught, which is minimal. Lots of damage, and if I'm right and you're wrong, it's only for the first occurance. By the third, we'd believe the terrorist's proclamations. It's missing the short term sizzle, but it's effective, and could have a cumulative effect similar to the bombers in Israel.
Aside from prairie or forest fires, urban arson could be a technique of terror. This block, that block, one dies here, four there, brings the risk home to all of us.
Of course if this started happening on a regular basis, every mosque in America would be burned the same week, and muslims would have to go into hiding.