John Mueller, a professor of political sceience at Ohio State University, writes an essay on this question in Foreign Affairs, published by the Council on Foreign Relations. The author never mentions Hezbollah which has a more robustly developed infrastructure in the US than AQ; which makes Iran's acquisition of nuclear weapons all the more dangerous given their state sponsorship of Hezbollah; particularly the intelligence support they provide to the terrorist organization.

The author also seems to be missing a rather large indicator happening all around the country with bulk purchases of 'cell phones'... see for example  The "controversy" surrounding bulk cellular telephone purchases.

He also overlooks the very evident jihadist threat in Europe ... which by extension would suggest the threat is potentially resident here as well.  Of course Europe and America are different, particularly the demographics of the Muslim population in Europe which per capita is much larger and un—assimilated. Analysts tend to focus on the latter issue, assimilation, as opposed to the former issue, population density for explaining these terrorist outbursts.

What we don't know about the terrorist threat in America is a function also of our still poor domestic intelligence capabilities and legal hamstringing... there are no doubt 'covert jihadist' and Islamist organizations that operate overtly in the US. 

Joe Myers   8 25 06