Flags at Half Mast Today Why?

I was wondering why all U.S flags are at half mast today? What am I missing? I don't remember flags at half mast on May 2011. I passed a military school today and I noticed all the flags being at half mast today, they usually do that out of respect. Maybe it's an important occasion? If someone read the news please let me know why the military is doing this today.

Flags are at half mast today in honor of Peace Officer Memorial Week. If you have a flag on your backyard fly it at half mast as a sign of respect. It is also Police Week. All flags are at half mast because of a Presidential proclamation which has been issued to fly flags this way today.
That's basically the explanation, all flags are at half mast in my town too, it's a national thing.