You really are an idiot if you think a dirty bomb in Chicago is not a viable scenario.
In fact, one pound of highly radioactive material, not even weapons grade, released in to the wind (over the Windy City) would wreak havoc all over the downtown core! Worse even, than the Towers falling! Do you really think it would be a big deal to get one pound of material into the US?
And this would be a great idea for the Iranians or any other muzzie terrorist faction they would sell powder to. No mushroom cloud. No missile track. Not even identifiers in the material itself to track back to Iranian production. You think Venezuela might help by loading it on a drug sub? I do. But just walking it across the border will work too.