Murder of Aiwa Matsuo

On July 26, 2014, Aiwa Matsuo, a 15-year-old high school student in Sasebo, Nagasaki Prefecture, Japan, was killed by a 15-year-old female classmate. The killing occurred in the suspect's apartment, where Matsuo was beaten with a metal tool, strangled to death, and then partially dismembered and decapitated. In Japan the case became known as the Sasebo high-school girl murder case.

The suspect — who was 15 at the time of the killing and turned 16 shortly afterward — allegedly posted details and photographs related to the crime on the Japanese message board 2channel. She reportedly told police, "I wanted to kill someone. I bought tools by myself." Prior to the killing, the suspect had attacked her father with a metal baseball bat, which led her father and step-mother (her biological mother having died the previous year) to move her into her own apartment when she began high school. It is believed she had previously attempted to "dissect" a cat and had possessed several medical textbooks.

The case had wider repercussions in Japan. Fuji TV cancelled a scheduled July 31, 2014 broadcast of a re-edited episode of the Psycho-Pass anime because it involved murder among teenage schoolgirls, in apparent response to the killing. The suspect's 54-year-old father publicly apologized to Matsuo's family in July 2014 regarding his daughter's mental health; he was later found to have died by suicide on October 5, 2014.

It was also revealed that a psychiatrist who had examined the suspect had contacted a child consultation center in Nagasaki Prefecture to warn officials that, if left untreated, she "could kill someone." This warning was not acted upon. In 2015, three officials at the Nagasaki Prefectural child consultation center were officially reprimanded by the prefectural government, which stated the center had "failed to fulfill its duties."

The case has drawn comparisons to other incidents involving juvenile violence in Japan, including the 2004 killing of 12-year-old Satomi Mitarai by a female classmate, also in Sasebo, and the 2015 killing of 13-year-old Ryōta Uemura in Kawasaki.