The worse potential specific security threat to mainland USA would be the detonation of a nuclear weapon in an American city. I think everyone is well aware of that risk, and it's in the back of everyone's mind.

There are numerous disaffected groups who would no doubt like to do just that, but they would have to acquire such a weapon first and then get the weapon onto (or very near) American soil to do it. The easiest way to do that would be to bring it in on a ship and detonate it in the harbour of some American city.

This is something that I'm sure is keeping American security forces busy on a fulltime basis...and let's hope to hell it never happens...because if it ded it would be a total disaster and it would only lead to even greater tragedies not long afterward. It would inevitably cause further wars to occur, as some country would get fingered with the blame (whether or not they really planned it).