If you follow their recent modus operandi, the plan will likely involve several trucks, well spaced out, set to go off nearly simultanously.
Are there any high-population areas that only have one bridge to the mainland? I want to say North Carolina has one such area, and perhaps Galveston Island in Texas, but are there others? They could take out a bridge and carry out other attacks on the island with fire and rescue personnel unable to get to the island in time.
You know, when you stop and think about it, there are all sorts of ways to hit us. That's part of the problem of being an open, and geographically huge, country.