Copyrighted materials need not bear the copyright symbol in order to be protected. Since 1978, all works created are automatically protected, regardless of whether the owner/author submits a copyright request; all works originally created before 1978 (whether registered or not) are also protected by copyright law. It is better to always assume a work is copyrighted and that you must obtain permission to use it (i.e. download it or share it). To learn more or search for specific works, visit the U.S. Copyright Office.