perfect crime
crimes that are undetected, unattributed to an identifiable perpetrator, or otherwise unsolved or unsolvable
en-US/pɚˈfɛkt ˈkɹaɪm/
en-GB/ˈpɜːfɪkt ˈkɹaɪm/
crimes that are undetected, unattributed to an identifiable perpetrator, or otherwise unsolved or unsolvable