Let’s look at it this way; if any terrorist group was to unleash any type of WMD against any American city then most of the Middle East would become a sheet of glass in no time. There would be swift retaliation and it would much more massive than they could ever believe. They would most likely be wiped off the face of the earth.
If we have a nuclear attack on Washington, DC, the first thing the people at the Nuclear Emergency Search Team (NEST) will determine is the source of the fissile material. This is not that hard to do since they can determine the impurities from the remains of the fissile material, which will trace where the material was originally manufactured. If they trace it to Iran, the city of Tehran and the holy city of Qom will soon find out the effects of a B83 nuclear bomb detonated at 1 MT yield dropped from a B-2 bomber.