I was in Japan when this case happened... it was huge news, dominating water cooler talk and the media for a long time:
Sakakibara Seito - Wikipedia, the free encyclopedia
A 14 year old boy killed an 11 year old boy and 10 year old girl, and took the boy's decapitated head to school, mounting it on the school's front gate with a note in the mouth. Police were stumped for quite a while as to who did it, and kept looking for an adult. Finally they realized it was this little kid...