I can think of 3 very easy to do and relatively inexpensie acts that would be effective. They would likely not be able to be stopped, if the persons involved was willing to be killed in carrying out the operations.
What puzzles me is they have the motivation, money, suicide volunteers and opportunity to do the same but it hasn't happened. Yet! Thank goodness. We may be operating on an "If in doubt,take them out mode." I hope so.
I'm sure other Freepers can think of some spectacular easy tagets that can be hit with a minmum of effort.
The terorists have the advantage of not worrying about escaping, or how to avoid capture.