Born on November 21, 1992, in Sasebo, Nagasaki Prefecture, Japan, Natsumi Tsuji was known to be an intelligent and creative student with an IQ of 143. She attended Okubo Elementary School, where she was recognized for her academic abilities and interest in literature and drawing. Despite her talents, Tsuji began exhibiting behavioral changes in the months leading up to the incident, including a fascination with violent media such as the film "Battle Royale" and the flash animation "Red Room." These interests, coupled with social isolation and reported online disputes, may have contributed to her deteriorating mental state. ​

On June 1, 2004, during the lunch break at Okubo Elementary School, 11-year-old Natsumi Tsuji lured her 12-year-old classmate, Satomi Mitarai, into an empty classroom under the pretense of showing her a new game. Once inside, Tsuji used a box cutter to slit Mitarai's throat and arms. After the attack, Tsuji returned to her classroom with bloodstained clothes and reportedly told her teacher, "I have done a bad thing." The teacher discovered Mitarai's body and contacted the authorities.

Upon arrest, Tsuji confessed to the murder, expressing remorse by repeatedly saying, "I'm sorry, I'm sorry." Initially, she did not provide a motive, but later stated that the attack was prompted by an online dispute where Mitarai had allegedly made derogatory comments about her weight and called her a "goody-goody." The case shocked the nation, leading to debates about the age of criminal responsibility in Japan and the influence of violent media on youth.

On September 15, 2004, the Nagasaki Family Court ruled to institutionalize Tsuji due to the severity of her crime, despite her young age. She was sent to a reformatory in Tochigi Prefecture for two years, with her sentence extended by an additional two years in 2006 following a psychological evaluation that diagnosed her with Asperger syndrome. In May 2008, authorities decided not to seek further extension of her sentence. ​