What would happen of US Govt. was destroyed in some sort of attack on DC?

Just rampant speculation here, don't call homeland security. We were speculating what would happen if something terrible happened like a nuke or biotoxin going off in Washington DC., either from a snuck in terrorist bomb or a rogue nation getting in a lucky shot with a missile. Everyone is dead from the president right on down to the secretary of dog catchers. Congress, senate, supreme court, everybody. Who is in charge then? Is there a system in place for progression of power?