What are the specific ethical considerations that need to be addressed when investigating a sexual assault?

I'm having trouble with the wording of your question. "Investigation" implies some police or legal event. But you're asking about ethical considerations. My impulse is to say, "There are no ethical considerations" but I could be misising something. If you want to cite the Ten Commandments in some sexual assault, you can. But the police don't enforce the Ten Commandments, they enforce local law. Perhaps the event that you want investigated involves two consenting adults. One (or both) are married. This violates ethics, for sure. But it's not illegal. Heck, in most places it's not even grounds for divorce. Assault is not legal, though it gets grey when you talk about specific "He said/she said" events. Maybe if you provide more info, you'll get a better answer. Take care!