More than likely it won’t be nuclear dirty bomb it will be a bio attacks.
Much easier to get into the country and much easier to hide once here.
A jar the size of a baby food jar could hold enough anthrax to contaminate every major city in the US.
According to a friend of mine in the air force the top brass tried to get Obama to bomb Saddam Hussein’s bunker where the bio weapons were believed to kept and seal it permanently before ISIS could get into it but Obama wouldn’t allow it and ISIS made a bee line to the bunker.
And the beauty of bio and dirty bombs is that the psychological value is so high that the weapon effectiveness can be nearly nil.
It simply has to be good enough to set off the radiation detectors. The panic and 24 hour news cycle will do the rest.