The police were very kind to us. We were never treated as suspects, and they were good about updating us as they discovered more information about the skull. Here is what we learned:
The skull belonged to a native American female, who died in her early 20's. They estimated the age of the skull to be between 200 and 400 years old. Wherever it had originally been buried must have been warm and dry because the skull was very well preserved--basically mummified. It was suggested that perhaps it was in a cave.
The skull was shipped to Florida, where there is a lab where they do genetic testing to determine which native American tribe it belongs to. Once identified, the plan was to return it to that tribe for proper re-burial. That is the last we heard of the skull.
How this skull got in our attic remains a mystery to this day. I will post my very plausible theory on how it got there next time. Stay tuned!