An investigator wants a written statement? Can I ask him why?
You need to speak with an experienced criminal defense attorney immediately. You should do this before you have any further contact with this investigator. There are many pitfalls that can come from making statements, and police investigators are not legally required to tell you the truth about what they are investigating or why. A defense attorney is someone who is actually on your side and who can give you advice about what is best to do under the circumstances.