This is a workplace investigation. It is a serious matter. You have been asked to assist us. We are trying to find out what happened here so we can advise management accordingly. We appreciate your time and cooperation.
I will not mislead you or lie to you. You should not lie to me. Lying in the course of the investigation can get you in serious trouble.
If you are an attorney, add: Please know that I am a lawyer. I represent the company, and I do not represent you. Anything we discuss here today is not subject to the attorney-client privilege, and I may share this information with company management.