As of 10AM this morning atleast a dozen police cars are surrounding Museum Square (401 K Street NW). The Mount Vernon Triangle CID passed along this note:
At this time, we know that someone had barricaded himself in the building, and a body is being carried out.
UPDATED [10/20/11 3:30PM]:
Final word is a male resident of Museum Square had a sword, held a hostage, and was taken into custody. No deaths reported.