I have not heard what caliber was used yet. There was some rumor that it was .308 or 30-06, both of those would be more capable of mass destruction from the shooter's "roost". While the more common .223 could travel that far it would not have much knock down power at long range. His 32nd floor position was an advantage to him because the shot is going down hill, the opposite direction it would be more difficult.
Just so you know, there is NO classification of guns that are called "assault rifles", that is a name made up by politicians. By definition any gun used for this attack IS an assault rifle because it is used to assault people. Assault rifle is interchangeable with "scary looking gun" to stir up emotion in the uninformed.
That was a fair question IMHO about this sick event.
On a side note I hear that the blood drives for the victims is going very well, but the more rare blood types are still needed. I'm taking my type O- but down to donate this Saturday. If any of you guys or gals are O- (O neg) they can use your help now!