Saari entered the school building through the basement on the morning of September 23, carrying a .22 caliber Walther P22 pistol and several bottles filled with gasoline. At approximately 10:40 local time, he began shooting in a classroom where students were taking a business studies exam. Around 200 people were in the building. Emergency services received the first call at 10:43, three minutes after the shooting began, from a student inside the classroom. In an adjacent room, a student named Sanna Orpana heard what she described as shooting and a kind of rumble, like tables falling down. Two of her classmates went to investigate; Saari fired at them. The remaining students hid under tables before fleeing upstairs. Between 10:45 and 11:00, Saari moved down a corridor and set fire to a language laboratory. He shot out every window along the school's main hallway. Eight of the ten people killed were female students, one was a male student, and one was a male staff member -- a teacher in his fifties. All the students killed were in their twenties, and all were Saari's classmates. Police found Saari at 12:13, still alive after a self-inflicted gunshot wound. He died hours later at Tampere University Hospital.

The shooting was Finland's second in less than a year. In November 2007, the Jokela school shooting had claimed nine lives, including the gunman Pekka-Eric Auvinen. The first such incident in Finnish history had been the 1989 Raumanmeri school shooting in Rauma, which killed two. Investigators found that Saari had been deeply influenced by the Jokela attack. In the months after it, he had begun dressing and behaving like Auvinen, though police ultimately found no direct contact between the two. Saari had posted a video on a Finnish social networking site in which he pointed a firearm at the camera, said "You will die next" in English, and fired four shots. This video was not available to police when they questioned him the day before the attack. A spokesperson later acknowledged that had they seen it, Saari would have been detained. He had also saved footage of the Columbine High School massacre in his YouTube favorites. The investigation revealed handwritten notes in his dormitory room stating he had planned the attack for six years.

The investigation eventually interviewed 200 people, none of whom said they had known about Saari's plans. The forensic evidence told its own grim story: Saari had fired at least 157 rounds. The victims sustained between one and seventeen gunshot wounds each, mostly to the head and upper body, with sixty bullet wounds tallied across all ten victims. The fires Saari set damaged some of the bodies, complicating identification. One discovery haunted investigators: the single male student killed was likely a close friend of Saari's. The two had spent an evening out together in February 2008. A photo had circulated online showing Saari jokingly pointing his finger at his friend's head. Saari had come to Kauhajoki from North Ostrobothnia. His early life had been marked by illness, slow growth, and the death of his brother when Saari was seventeen. He had been bullied in school, suffered from anxiety and panic attacks, and was expelled from the Finnish Army after one month of service for firing his weapon during a training exercise against orders.

Prime Minister Matti Vanhanen described September 23 as a tragic day and traveled to Kauhajoki to meet with students. A crisis meeting convened that same day with government ministers, parliamentary group leaders, and police officials. In the days that followed, police reported receiving a surge of tips about threatening content online -- suspicious photos, videos, and chat room messages. Bomb threats and other menacing messages circulated among students nationwide. Police Commissioner Mikko Paatero announced that Finnish police would increase their monitoring of YouTube and social media. When asked whether similar attacks could happen again, he replied: "I sadly fear it's possible." The shooting forced Finland to confront uncomfortable questions about its firearms culture, its mental health systems, and the gap between identifying a potential threat and having the legal tools to act on it. The police had been in Saari's home twenty-four hours before the attack. They had seen him, spoken with him, and searched his belongings. The law, as it stood, had not given them enough to stop what came next.