Looks like a well placed mortar round from a 80-120mm... whether it was a planned hit, or unfortunate circumstance remains to be seen (kinda like the Scud hit on the barracks back in '91).
I'm anxious to see the results of the investigation into the attack - could mean that there is a plant, or the insurgents have gotten their hands on some good targeting gear - for such a possibly pin-point attack.
