Tomohiro Katō (加藤智大, Katō Tomohiro?) (born September 28, 1982) grew up in a suburban home in Aomori, Honshu. His father was a top manager in a financial institution. Katō's grades were considered to be exceptional in elementary school and he was a top track athlete. He entered Tsukuda Junior High School and became president of the tennis club in middle school. He started to act violently at home after enrolling at Aomori Prefectural Aomori High School, an elite high school. Katō was unpopular with his classmates and his class academic ranking fell to 300 (of 360 students). He failed entrance examinations for the prestigious Hokkaido University, eventually training as an auto mechanic at Nakanihon Automotive College. He was hired as a temporary worker at an auto parts factory in central Shizuoka Prefecture, though he was recently told that his job was going to be cut at the end of June.
The suspect Katō was arrested red-handed of attempted murder after a police officer spotted him stabbing a woman. On June 10 the police sent him to Tokyo District Prosecutor's Office. On June 20 the police arrested him again on suspicion of murder of the seven victims. On the same day the Prosecutor's Office withheld action on him for the first suspicion. While being positive about his capacity to hold the criminal liability, the Prosecutor's Office decided by June 20 to demand that his detention for a psychiatric test be authorized by the Tokyo District Court.
Katō was cooperative during the inquiry but unapologetic, and cried at times. Police seized from his apartment empty packages of knives, their receipts, and one club.
The massacre also sparked many conversations in Japanese blogs when it was discovered that two Ustream users had broadcast live video streams of the tragedy, attracting a viewership estimated at between 1000 and 3000 people. No known recording has been saved of the videos, although the event has been written about in many Japanese blogs and online IT magazines.