With the weapon being used, a head shot would be stupid. The goal here is Terror, not just killing. Best to increase odds of a hit, then just take someone down.
First attempt that missed could have been a range check on a sucky weapon. Brass left behind was meant to be seen.
Snipers dont talk to their enemy. This person wants to be known, and he is. If he did everything right from a sniper perspective, then everyone would be thinking terrorism and looking outside rather than in.
I think that they should check the records of who is sniper trained nationwide, and then cross-check that list with who has served some time in prison and converted to Islam. Home-grown recruits for Al Qeda are plentiful inside the walls - Patrice Ford kind of guy with some sniper training.
Or maybe it is just some @ssholes from The Brady Bunch (Sarah, et al.)