Washington Monument at half-mast

That day when I was sitting near it, sitting outside while I worked on a paper or proposal, all of the flags around the base of the Washington Monument were at half-mast, and I have no idea why.  I've noticed this recently, that I much more often notice flags at half-mast than raised all the way, and I suspect this may be about the increasing connectivity of our world and the fact that there's always a tragedy happening somewhere to be noticed.