Can't remember what alphabet agency it was, but as I was scanning the sunday news shows, one of them said it was not only possible but highly likely. In New York city alone, he said they have stopped 12 attempted terrorist acts this year alone. Eventually one of them is going to slip through. ICIS put out a notice yesterday that Washington DC was the next target. Its a large gun free zone, so I guess that might be why they picked it along with other reasons.
DC is not anything near a gun free zone. Sure, regular scmucks like us are not armed but there are copious amount of armed police of various agencies all over the government buildings and transportation hubs. Now, I'm sure just a few blocks outside the green zone in the ghetto its a free for all. But any of the high profile targets in DC are well defended.