The US Capitol was on the target list, but it's a very big area. These guys were not only trying to bang the buildings, but kill the contents!
Remember, there was an anthrax attack already on the way.
I agree there was an Anthrax attack on the way but I believe the letters to the Senators were postmarked 3 weeks after 9-11. They may have been originally attended for a different group...or if the same group, no matter where the Senators were, the mail was going to be forwarded to them. The fact is: The letters got through.
Thinking about it, we could have had anthrax turning up in "Secret Locations" if the Capitol and everything around it had been hit. I believe BOTH planes heading to DC had the same target. Boom, boom....just like NY.