The critical parts of the electrical grid are pretty much unprotected, the same qirh water sources. Given some of the recent events, many of the police aren't much interested in getting involved unless they have a formidable force with them, and all the toys.
They’re so many things a determined group of terrorists could pull off, many without guns.
Depending on what type of weapons they’ve managed to acquire determines what type of attacks they could pull off.
In major cities given how many have cut back on police forces, they could overwhelm the police which normal everyday criminals would quickly join in, it would be chaos in major cities.