The Arizona Republic has unveiled a June 25 FBI memo, warning that, in the AR's paraphrase, "a senior al-Qaida detainee told federal investigators he had developed a plan to set midsummer forest fires in Colorado, Montana, Utah and Wyoming." Western vulnerability to a group arson attack has always been an irrational worry of mine, given the weather conditions, number of people living near forests, and so on … though I suppose hikers would begin to notice if obvious non-backpackers kept showing up and sniffing around for the driest brush.
The article also contains this historical nugget:
America's national forests have been targeted before. On Sept. 9, 1942, a plane launched from a Japanese submarine flew over Oregon's forests dropping incendiary bombs. The plan was to cause massive conflagrations in the forests that would be hard to fight because of their size and the lack of manpower. But the weather didn't cooperate, and the fires the Japanese set didn't spread very far.