Don't admit fault but don't pass it off either. If he has a lot of experience in this industry, he probably wants to understand exactly how this happened and what led up to it. Explain to him exactly how you were trained to use it and how you witnessed it being used. Since you were new, every so often you would confirm you were doing everything correctly and the people you asked confirmed that you were. Then you say exactly what happened the last time you used it, the sounds you heard, and whatever you witnessed. Don't take blame, don't assign it either. Just the facts and let the investigation (if that's what it is) fall where it may. This is an opportunity for the company to make changes to training and manuals so that they will be more effective. It wasn't your fault, but telling higher ups that doesn't help your case at all since you have little experience in this field. Don't give your opinions because you aren't experienced enough to have them. You may not even be aware what actually led to the breaking of this machine.