That doesn't discount the possibility of assembling such a bomb from radioactive materials within the US. I really think that a group of islamists gathering radioactive material inside the US might raise the alarm. The bomb builders still face the problem of a lethal dose of radiation.
I don't think this is a practical weapon tha can be used in the US.
British authorities have said they believe suspected Islamic terrorists were harvesting radioactive materials from household smoke detectors.One source of rad mat. Many others too. Including some bright kid and his collection of Coleman Mantles.Time magazine reported that following the arrest of a suspected terror cell in August, British police discovered large amounts of smoke detectors that contain small amounts of americium-241. They believe the group planned to use the radioactive element to construct a primitive dirty bomb.
- The Washington Times (via http://www.blogsofwar.com/archives/2004/10/04/uk-terror-cell-was-building-dirty-bomb-from-smoke-detectors/)
And what if it arrives by boat?
I agree, unless an outside party(country) is helping the terrorist, which if it is nuke of any sort, chem or bio would be the case. Thus the reason for the warning by the Bush administration, at the beginning of the War on terror, to any country helping the terrorist that they would be next on the list to be taken out and that our response to any of these weapons would be nukes. This is a bigger game than just islamofasicim.