The article said the bushmaster was found in the school with Lanza. it would be a safe bet to say he used it.
The problem is that we don't know for certain that he actually used the rifle or he merely had it with him. Why CSP has not released this particular piece of information is beyond me, unless they have been told to not release it. And the only reason for this would be political; that is, if the weapons used were, in fact, the handguns and not the Bushmaster, much of the wind would be taken out of the sails of the movement to ban military-style rifles.
I may very well be wrong here, but that is the only reason I can think of. And just FTR, I think that most conspiracy theories are nonsense.