The Sasebo slashing (佐世保小6女児同級生殺害事件? 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. 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 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, she 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. She 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.
The killer 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 whom was murdered, Girl A the killer, 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".
- "Japanese schoolgirl kills classmate". The Sydney Morning Herald. 2004-06-02. Retrieved 2008-06-06.
- Yamaguchi, Mari (2004-06-02). "Japanese girl accused of killing classmate". The Independent. London. Retrieved 2008-06-24.
- "Girl says internet spat prompted slaying". China Daily. 2004-06-04. Retrieved 2008-06-23.
- "Japanese girl stabbed to death in school". China Daily. 2004-06-02. Retrieved 2008-06-06.
- ネットに加害女児の顔、名前 長崎地方法務局が削除要請. Nagasaki Shimbun (in Japanese). 2004-06-03. Retrieved 2008-06-23.[dead link] Internet Archive copy.
- "Sixth-grader kills her classmate, 12". The Japan Times. 2004-06-02. Retrieved 2008-06-23.
- "Japanese girl, 11, cuts friend's throat". The Age. 2004-06-02. Retrieved 2008-06-23.
- "Japanese Girl Fatally Stabs A Classmate". The New York Times. 2004-06-02. Retrieved 2008-06-23.
- "Japan stunned by schoolgirl crime". CNN. 2004-06-02. Retrieved 2008-06-23.
- "Japanese girl, 11, kills classmate by slitting her throat". Scotland on Sunday. 2004-06-02. Retrieved 2008-06-24.
- "Japan in shock at school murder". BBC News. 2 June 2004. Retrieved 2007-09-19.
- Brooke, James (2004-06-03). "Internet Messages Cited In Girl's Killing". The New York Times. Retrieved 2008-06-23.
- "An 11-year-old Japanese girl to be placed in juvenile center over classmate's slaying". Boston Herald. Associated Press. 2004-09-15. Archived from the original on 2006-05-04. Retrieved 2007-09-19.
- "11-year-old killer institutionalized". The Japan Times. 2004-09-25. Retrieved 2008-06-23.
- "Girl who fatally stabbed classmate to have freedom restrictions lifted". Mainichi Daily News. 2008-05-29. Retrieved 2008-06-06.[dead link]
- "Father of murdered Sasebo girl speaks on lifting of attacker's freedom restrictions". Mainichi Daily News. 2008-05-30. Retrieved 2008-06-06.[dead link]
- Kusanagi, Atsuko (2005-11-24). "第七章 「コミュニケーション能力」と「エピローグ」" [Chapter 7: "Communication Ability" and "Epilogue"]. 追跡！「佐世保小六女児同級生殺害事件」 [Tracked down! (The case of the Homicide Committed by a Sasebo Elementary Grade 6 Girl Against Her Classmate)] (in Japanese). Japan: Kodansha. pp. 93–110, 214–228. ISBN 4-06-213041-6.
- Watson, Nicholas (June 21, 2004). "Violent crime prompts debate over age of legal responsibility in Japan". Publique!. Retrieved 2007-09-19.
- "Japan stunned by schoolgirl stabbing". The Daily Telegraph. London. 2004-06-02. Retrieved 2008-06-23.
- Faiola, Anthony (August 9, 2004). "Youth Violence Has Japan Struggling for Answers - 11-Year-Old's Killing of Classmate Puts Spotlight on Sudden Acts of Rage". The Washington Post. p. A01. Retrieved 22 June 2012.[permanent dead link]
- "Japan killing comments spark row". BBC News. 2004-06-04. Retrieved 2008-06-23.
- "School slaying a sign of gender equality: minister". Taipei Times. 2004-06-05. Retrieved 2008-06-23.
- "Ministers told to watch their mouths". The Japan Times. 2004-06-12. Retrieved 2008-06-06.
- "Using computers for long hours may prompt children to behave violently, neurologists says". Medical News Today. 2004-06-23. Retrieved 2008-02-07.
- "川島 隆太 氏 インタビュー「道を拓く- Frontiers -」" (in Japanese). Science Portal. Retrieved 2008-05-31.
- 殺害手口、参考の可能性 ネットの物語掲載サイト. Nagasaki Shimbun (in Japanese). 2004-06-09. Retrieved 2008-06-23.[dead link] Wayback Machine copy.
- "Japan schoolgirl killer 'sorry'". BBC News. 2004-06-03. Archived from the original on 13 February 2007. Retrieved 2007-01-12.
- "Murdered girl's classmates get blank page for killer in graduation album". Mainichi Daily News. March 18, 2005. Retrieved 2006-03-25. (link inactive since last access date)
- "Slain Sasebo girl awarded posthumous graduation". The Japan Times. 2005-03-18. Retrieved 2008-06-23.