I wonder why they figure Washington would be the primary target.
Politicians are easy to replace, they should be replaced at each election anyhow.
The business leaders are not so easy to replace, and technical experts like university researchers are even less replaceable.
Surely any sane terrorist would prefer to damage the economy and cause political unrest instead of eliminating the people they are trying to negotiate with.
When you want to harm a nation, destroy it's capital and its government leaders. It's fundamental.
The current crop of terrorists don't seem interested in negotiating.
Business leaders are replaced all the time and they don't all live and work in one city. Technical experts -- we can issue more visas to those who can't telecommute from India. They aren't concentrated in one area, either.
The only other target that comes close to the destruction of the federal government infrastructure would be New York City and it's financial infrastructure.
Congress would not be easy to replace. There is not a uniform system for replacing members now when seats open up due to death or resignation. Since 9/11, this has been the subject of legislative proposals and hearings but has not been settled.
The Supreme Court can only be confirmed by the Senate which no longer would exist. In a weekday blast scenario, POTUS and the VPOTUS may not survive.