This is shocking on so many levels. First of all just the horror of losing so many good men on a base in their home country. Then the shooter being an officer (OK, not from the fighting arms, but still wearing a uniform). a doctor for god's sake!
And I am sure just the names Fort Hood and Texas have some resonances of their own for a lot of people..
It looks right now like there was just one shooter, and he had some issue with being posted to iraq. I am sure that explains nothing, but that would point to "lone nut goes berserk". On the other hand, if it turns out to have any connection to spontaneous Jihad syndrome or even worse, if it was a planned act by more than one person then we will surely be talking about many other things.
Just shocking.