The cynic in me wants to say that you could be very concise with the language you choose to use when communicating with law enforcement. You are not obligated to hand-over the footage, but I can understand your willingness to assist. My concern is that, now that you know your footage may contain evidence, or if it does actually contain evidence (which you may not know is material to their investigation), then if they believe you are withholding it intentionally, you might be accused of evidence tampering.

I'd not volunteer any further information until they reach out to you again, then I would say something to the affect of "I am unable to provide this footage as, to the best of my knowledge, I do not know to extent to which it exists and do not know how to it can be retrieved. This information is with Google."