I'm guessing the Capitol building or White House.
I think they were going for a Trifecta:
Our Economy - WTC
Our Military - Pentagon
Our Government - ?
I think you're right, nully...that makes the most sense.
In London, during the tube bombings, the authorities also said they noticed a geometric pattern to their bombings....though what you suggest, in this case, is obviously more likely their target being the White House (or Capitol Bldg.)