Who owns the copyright on music created with AI?

A few court cases have decided that works created solely using AI can't be copyrighted. However, the copyrightability of works where some human action has occurred is still an open question.

According to the U.S. Copyright Office, copyright is granted only to works that involve a human author.

So, does the work belong to:

1) no one

2) the human user

3) the AI tool creator

4) joint ownership

5) copyright holders of training data

6) public domain