IMO the most deadly way for terrorists to attack a major American city would be a dirty bomb or large chemical or bio weapon on a ship-borne container. I don't want ANY Islamic country having control over any of of ports for that reason - all it takes is a couple of sleepers working their way up in the management structure to be sure to not find the weapon when it arrives.
I've got news for you. A terrorist who uses a dirty bomb or large chemical or biological weapon to attack a major U.S. city is far more likely to develop such a weapon right here in the United States.
Maybe I missed the part of that story about the "Abu Dhabi, UAE" postmark on those anthrax letters.