At 12:33 p.m. JST, a man hit a crowd with a truck, eventually killing three people and injuring two; he then stabbed at least 12 people using a dagger (initially reported as a survival knife) killing four people and injuring eight.
The Tokyo Police arrested , 25, on suspicion of attempted murder. The suspect, dressed then in a black T-shirt with a jacket and off-white trousers, was a resident of Susono, Shizuoka. He was held at the Manseibashi Police Station. Two days later on June 10, he was sent to the Tokyo District Public Prosecutor's Office. He was later re-arrested by the police on June 20 on suspicion of murder.
According to police and hospital officials, six of the seven who were killed were men, including Kazunori Fujino and his friend Takahiro Kamaguchi (both 19), Katsuhiko Nakamura (74), Naoki Miyamoto (31), Mitsuru Matsui (33), and Kasuhiro Koiwa (47). A woman, Mai Mutō (21), was also killed. Communication records showed that Mutō probably made an emergency call for police from her mobile phone#Noun, though she left no message. Later that day, a makeshift memorial was created by passersby.
Katō reportedly did not get along well with his parents, and seldom returned home. An interview with Katō's brother revealed that his parents had put immense pressure on their studies and performance, ordering that their homework be redone to standards in order to impress teachers in school and recalling one incident where Katō was made to eat scraps of food from the floor. Another neighbour described Katō being punished by his parents, who made him stand outside for hours in deep cold during winter. Previous online postings before his announcement of the attack contained sharp criticisms towards his own upbringing. Under debt and believing that his family had given up on him, Katō unsuccessfully attempted suicide in 2006 by ramming his car into a wall.
The suspect apparently posted messages from his mobile phone to a Web site "Extreme Exchange, Revised (partly erroneously reported as 2channel), which has since been shut down by authorities, revealing his intentions, posting his last message via his mobile phone 20 minutes before the attack. A police official stated the first message read, "I will kill people in Akihabara." Other messages he is alleged to have posted include, "If only I had a girlfriend, I wouldn't have quit work," "I would never have become addicted to my mobile phone. Anybody with hope couldn't possibly understand how I feel, and "I don't have a single friend and I won't in the future. I'll be ignored because I'm ugly. I'm lower than trash because at least the trash gets recycled." It also referred to "a stabbing spree in Tsuchiura. Commentators referred to the incident, based on the messages, as another case with the phenomenon of Hikikomori or Internet suicide. Later messages revealed his plan to use a vehicle until it became inoperable, and then to use a knife to continue the attack on foot. He waited out for Chūō-dōri to close at noon to vehicular traffic before commencing the attack.
The suspect Katō was arrested red-handed on suspicion 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. The police on June 20 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.
Katō erased all contacts and communication records from his mobile phone just prior to the attack, the purpose of which he confessed was to avoid annoying those around him. Katō later said that he posted the online messages hoping that police would take notice and stop him.
The knives were reportedly purchased two days before the attack from a military supply shop in Fukui at about 12:40 p.m. Katō spent about 20 minutes in the store, purchasing a telescopic baton and a pair of leather gloves while the store closed-circuit television captured him talking to and laughing with the salesman and demonstrating stabbing motions. Katō came to Akihabara a day before the attack to sell his personal computer and some software to raise money to rent the truck.
Japan's Code of Criminal Procedure limits police custody of the arrested to 48 hours, and limits prosecutor's time to 24 hours before filing charge against or requesting detention of or releasing the transferred suspect; the term of detention (begining with the day of requesting) is limited to 10 days, whose extension on unavoidal condition is limited to 10 more days as to suspicion like murder.
Konami canceled three launch events of Metal Gear Solid 4: Guns of the Patriots in Tokyo, with the "safety of participants in mind" as a result of the attack. Katō's parents gave an apology to the victims in a television interview.
The massacre also sparked many conversations in Japanese blogs when it was discovered that two Ustream users (Lyphard and kenan) had broadcast live video streams of the tragedy, attracting a viewership estimated at between 1000 and 3000 people. No recording has been saved of the videos, although the event has been written about in many Japanese blogs and online IT magazines.
The current Super Sentai series at the time of the attack, "Engine Sentai Go-onger" (2008) featured transforming daggers as part of the heroes' standard personal sidearms, called Switch Funshaken Rocket Dagger(s) (in reference to their rocket-shaped themes). After the attack, which occurred the day immediately after the Rocket Daggers made their debut in the series, both Bandai [the company that makes the toy versions of the weapons] and Toei [the company that produces the TV series] changed their names to "Switch Funshaken Rocket Booster(s)" and re-designated them as "swords" rather than "daggers" out of respect for the victims of the attack, and to lessen any trauma toward the 6-8yrs audience that the tokusatsu franchise regularly targets.
A few days after the attack, police arrested several people who stated their intention to make copycat killings elsewhere in Japan, including one case who made his intentions known to popular message board 2channel. Also on June 22, three women were injured by an unknown suspect at Osaka Station. A 38-year-old woman confessed of two cases. First actual stabbing event that happened after the Akihabara massacre. Between June 8 and June 23, 12 people have been arrested, and five people warned for the threatening messages. The 17 people involved in the threatening are age ranged 13 to 30 years old. On 26 June 2008, Police overpowered and arrested a man who was found to have a knife in possession in Akihabara.