There are three primary ways this can go that very plausible cases can be built for:
1) Large cities are attacked not unlike Paris, but on a larger scale.
2) Small towns are randomly attacked across the country, enough so as to spread fear in a “nowhere is safe” manner being most small town folks just assume staying away from the large cities is all they have to do.
3) The grid gets taken down, if not in all of the country, covering a large enough geographic area/large enough segment of the population so as to cause major damage, and many deaths dependent upon how long it takes to bring it back up.
Which way it will go, or when is beyond my ability to know but it is coming in a sooner rather than later timeframe. It isn’t years away.