The Sasebo slashing (Japanese: 佐世保小6女児同級生殺害事件 Hepburn: Sasebo shōroku joji dōkyūsei satsugai jiken) was the murder of a 12-year-old Japanese schoolgirl, Satomi Mitarai (御手洗 怜美 Mitarai Satomi), by an 11-year-old female classmate named Natsumi Tsuji (辻褄 夏美 Tsuji Natsumi). The murder occurred on June 1, 2004, at an elementary school in the city of Sasebo in Nagasaki Prefecture. The murderer slit Mitarai's throat and arms with a utility knife.
Reactions to the incident have included Internet memes and a discussion of lowering the age of criminal responsibility in Japan. The killer's real name has not been released to the press, as per Japanese legal procedures prohibiting the identification of juvenile offenders. Japanese police referred to her as "Girl A."  The Nagasaki District Legal Affairs Bureau cautioned internet users against their revealing her photos.
On June 1, 2004, the 11-year-old schoolgirl named Natsumi Tsuji, murdered her 12-year-old classmate, Satomi Mitarai, in an empty classroom during the lunch hour at Okubo Elementary School in Sasebo. She then left Mitarai's body and returned to her own classroom, her clothes covered in blood. The girls' teacher, who had noticed that both were missing, found the body and called the police.
After being taken into custody, Tsuji was reported to have confessed to the crime, saying "I am sorry, I am sorry" to police. She spent the night at the police station, often crying, and refused to eat snacks she was offered. Tsuji initially mentioned no motive for the killing. Shortly afterward, she confessed to police that she and Mitarai had quarreled as a result of messages left on the Internet. She claimed that Mitarai slandered her by commenting on her weight and calling her a "goody-goody".
On September 15, 2004, a Japanese Family Court ruled to institutionalize her, putting aside her young age because of the severity of the crime. She was sent to a reformatory in Tochigi Prefecture. The Nagasaki family court in 2004 originally sentenced her to two years of involuntary commitment, but the sentence was extended by two years in September 2006. On May 29, 2008, local authorities announced that they did not seek an additional sentence.
The killing provoked a debate in Japan whether the age of criminal responsibility, lowered from 16 to 14 in 2000 due to the 1997 Kobe child murders, needed to be lowered again. The killer was considered to be a normal and well-adjusted child before the incident, which made the public more anxious.
Members of the Japanese Diet, such as Kiichi Inoue and Sadakazu Tanigaki, came under criticism for comments made in the wake of the killing. Inoue was criticized for referring to Girl A as genki (vigorous, lively), a word with positive connotations. Sadakazu Tanigaki was criticized for referring to the method of killing, slitting of the throat, as a "manly" act.
Tsuji became the subject of an Internet meme on Japanese web communities such as 2channel. She was nicknamed "Nevada-tan" because a class photograph showed a girl believed to be her wearing a University of Nevada, Reno, sweatshirt.
Akio Mori cited this case in support of his controversial "game brain" theory, which has been criticized as being nothing more than superstition. The killer was reported to be a fan of the death-themed flash animation "Red Room", a claim used in support of the theory. It was also known that she had read the controversial novel Battle Royale and had seen its film adaptation, which centers on young students fighting to the death.
At the March 18, 2005, Okubo Elementary graduation, students were given a graduation album with a blank page in honor of Mitarai's death on which they could put pictures of Mitarai, Girl A, or class pictures containing both girls, Mitarai was posthumously awarded a graduation certificate, which her father accepted on her behalf. The killer was also awarded a certificate, as one is required in Japan in order to enter junior high school and the school believed it would aid her "reintegration into society".
- Sasebo schoolgirl murder, a 2014 murder in Sasebo
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