One dirty bomb could be concealed in a lead (Pb) encasement and positioned near the New York Stock Exchange or in Washington D. C.'s Congressional District, then once detonated, the prevailing winds which would have already been factored in to cause the most widespread devastation, would then carry a massive amount of radioactive dust particles --- over an extremely large metropolitan area.
There would be a cascading chain of panic and general mayhem, with the dead and dying continuing for months and years.
If the American public fails to accept this type of outcome, then there would be a spiraling-down into mass chaos.
http://www.cityofmemphis.org/pdf_forms/dirtyBlast.pdf