Today is Peace Officers Memorial Day and that’s why the flag is at half staff (Had to search, I didn’t remember). Small comfort probably to those that have lost someone in the line of duty. Also a reminder to the rest of us that though we may have bad days at the office, it’s never about having someone trying to kill us.
The Wikipedia entry for half staff is interesting – half mast is reserved for naval vessels and you don’t have to fly the flag at half staff. There are a few sites out there that show when/why the flag is supposed to be at half staff, the one that seemed best to me was http://halfstaff.org/.