It certainly looks like it might be a bullet strike, but I have seen rocks thrown by lawnmowers do similar damage. The story keeps alluding to multiple shots, but there doesn't seem to be any evidence of multiple strikes on the building. I don't think it was a structural failure brought on by previous damage, though. Some object struck the window. They need some physical evidence.

.. it looks like a bullet hit. The circular spread of the cracks seems to say something hit at high velocity. The center hole looks tiny, pellet gun perhaps? The shattering of the window might have made a loud bang...

small caliber like a .22. Spontaneous glass breakage apparently does happen.
http://en.wikipedia.org/wiki/Spontaneous_glass_breakage

center hole looks tiny.

I would think any caliber over a .22 would have blown out the window. I think people heard the initial strike, not the shot, and then when the window started cracking they mistook the sound for additional shots fired. When cracks propagate like that the noise can be surprisingly loud, and takes place somewhat sporadically.

when it pass through glass cleanly.

If it was a center fire or rim fire cartridge the bullet would be found in a wall .

If the investigators said it wasn't a bullet I believe them.

bb gun might do it, a high powered type

Small caliber, could have even been a slingshot though. If the investigators say it broke itself, I wouldn't believe anything else they had to say about it.

that it is a BB from a higher powered air gun or a pellet at a lesser power.

The damage appears identical to what a local store received some years back:

-Small center hole
-Rounded divot on interior, approximately quarter-sized diameter
-Entire window breaks but remains intact (if it breaks at all)
-no signs of damage to interior from projectile. (the bb/pellet actually bounced off and is outside the building)