I can only tell you what I'd personally do in such a situation. I'm not the guy for legal advice.

I'd file a report with the date and time you noticed the loss, the actual date and time of the transaction, transaction info, and your info. I don't know if it's a good idea to include the keys or not. On the one hand, you'd have a time-stamped report in case those keys are leaked in the future. You'd have evidence of knowing the key sooner than everyone else. Maybe ask an attorney if you know one. DA maybe