Why is the flag at half staff today?

CC image – flickr – SoulRider.222

A patron asked why the American flag outside our building was at half staff today. I didn’t know off-hand but I guessed that it was for the victims of the Oak Creek, WI shooting. After doing a quick Internet search, I found a website, Half Staff American Flag Notifications, that can tell you why and when the flag is at half staff.
I’m surprised that the federal or state government doesn’t have a notification site for this. At least this site adds the official proclamation by the President so you know it’s official.