I don't know of any standard question sets, but all my investigations pretty much run through the same information:
1. Please describe to me, in your own words, what happened. Include where it happened, who was involved, when it happened, and how it happened.
2. Were any witnesses present?
3. Does anyone else have knowledge of this situation?
4. Was this the first incident?
5. Who initiated said incident?
6. How did the situation end?
7. Will you put a statement in writing?
Hope this helps get you started. I find there's often not one way to do these things - questions are going to come up as you learn more from the interviewee.
"You are you. Now isn't that pleasant?" -Dr. Seuss