On Tuesday, he walked into a vocational college, the School of Hospitality, and opened fire, killing 10 people and burning their bodies with firebombs before shooting himself fatally in the head. At least two other people were wounded.
The rampage bore eerie similarities to another school massacre in Finland last year in which an 18-year-old gunman killed eight people and himself. Both gunmen posted violent clips on YouTube prior to the shootings, both were fascinated by the 1999 Columbine school shootings in Colorado, both attacked their own schools and both died after shooting themselves in the head.
The latest shooting raised questions about whether police could have stopped the bloodshed, and although there was little initial debate about gun control, Prime Minister Matti Vanhanen said the government may consider restrictions on privately held semiautomatic weapons.
There are roughly 1.6 million firearms in private hands in Finland, a nation with deep-rooted traditions of hunting in the sub-Arctic wilderness. The country's 650,000 licensed gun owners - about 13 percent of the population of 5.2 million - include hunters, target shooters and gun collectors, and Finland ranks in the top five in civilian gun ownership per capita along with the United States, Yemen, Iraq and Switzerland.
Finnish media identified the gunman as Matti Juhani Saari, a 22-year-old student at the school, which offers courses in catering, tourism, nursing and home economics. Police declined to identify him, saying he did not have a previous criminal record.
Witnesses said panic erupted as the masked gunman, dressed in black and carrying a large bag, entered the school just before 11 a.m., and started firing in a classroom where students were taking an exam.
"I heard several dozen rounds of shots, in other words it was an automatic pistol," school janitor Jukka Forsberg told broadcaster YLE. "I saw some female students who were wailing and moaning and one managed to escape out the back door."
About 150 students were inside the school, 180 miles northwest of Helsinki, when the shooting began. Students and teachers were evacuated - some reportedly fleeing down a nearby river in row boats - as police arrived.
Jari Neulaniemi, the officer heading the investigation, said the attacker was armed with a .22-caliber pistol and firebombs that were used to start several fires. Many of the bodies in the school were burned beyond recognition, he said.
The gunman left two handwritten messages at the dormitory saying he had planned the attack since 2002 and that he hated the human race, Neulaniemi said
A video clip posted on the Internet by the alleged gunman showed him pointing his gun to the camera and saying "You will die next" before firing four rounds.
Finnish authorities did not confirm exactly what YouTube clips were linked to the shooter. But in one clip posted by a 22-year-old "Mr. Saari," a black-clad man with thinning blond hair fired several shots in rapid succession with a handgun at a shooting range.
The posting was made five days before the shooting and the location was given as Kauhajoki. It included menacing lyrics from a German electro-industrial band Wumpscut: "Whole life is war and whole life is pain. And you will fight alone in your personal war."
"Mr. Saari" also posted at least three other clips of himself firing a handgun in the past three weeks.
YLE said he also made postings on Finnish Internet chat rooms, saying he lived alone with his cat and that his interests included shooting, playing drums, heavy metal music and horror movies.
On Monday, police brought in the gunman for questioning about the Internet video. They said they released him because he hadn't broken any laws and was not deemed a threat to others.
Interior Minister Anne Holmlund, who heads the police department, said nothing indicated there had been negligence on the part of the police.
"It's clear that we have to carefully go through what should have been done and if we could have avoided this situation in some way," Holmlund said.
Laura Lodenius of the Peace Union of Finland disagreed.
"Police interrogated this man on Monday but did not think they had the legal powers to withdraw his gun license. That really shows that there is something wrong," Lodenius said.
Still, some said Finnish police should not be blamed.
"It's unfair to expect police to be able to be clairvoyant ... and know whether or not someone making a video intends to commit harm consistent with that video," Interpol Secretary General Ronald Noble told The Associated Press in a telephone interview.
"People will be second-guessing," Noble said. "Knowing now that he killed these people, you look at the video - it looks even more frightening. So now you say, 'How is it that they let him go? How is it that they didn't arrest him? How is it that they didn't put him in observation in a psychiatric ward for three days?"'
He suggested that some countries need regulatory codes that would allow firearm licenses to be revoked for people who have shown signs they might pose a threat with their guns.
"Then police can say, 'OK, we don't have enough to arrest you but we are certainly going to take your firearms away,"' he said.
Gun control activists attacked the Finnish government, saying it should have restricted gun laws after the 2007 shooting at Jokela high school. At the time, the government promised to raise the minimum age for buying guns from 15 to 18, but that measure has not yet been passed.
"If public safety had been put at the heart of Finnish gun laws after Jokela, then today's terrible events may have been prevented," said Rebecca Peters of the London-based International Action Network on Small Arms.
President Tarja Halonen said at the United Nations that the shooting showed the need for the older generation to watch over the activities of the young on the Internet and to renew discussions about handgun legislation in Finland.
Foreign Minister Alexander Stubb described Finland's gun laws as "very strict."
"I'm sure we will now in Finland start a discussion also about the difference between what can be called hunting rifles and handguns," he said.
Vanhanen called an emergency Cabinet meeting, saying he and other ministers planned to go to Kauhajoki - a town of 14,000 - on Wednesday.
"We have experienced a tragic day," Vanhanen said, expressing condolences to the families of the victims and declaring Wednesday a day of mourning.
A fellow student described the gunman as "a regular and calm guy."
"Nothing outstanding. He had lots of friends. Nothing that would have given an idea that something like this would happen," Susanna Keranen told AP Television News outside the school.