Police in the town of Hyvinkää, some 50km north of Helsinki, say a young man dressed in military fatigues began shooting with a rifle from the roof of a building in the city centre at 1:53am Saturday.
An 18-year-old woman was killed. Another victim, a 19-year-old man, died later in a hospital. Seven other people have been hospitalised with gunshot wounds, including a 23-year-old woman police trainee, who has critical injuries.
The suspect was detained at 7:45am. Police say he is an 18-year-old from Hyvinkää with a “nearly clean” record but no gun license.
According to Police Inspector Markku Tuominen, he acquired the weapons used in the shootings from acquaintances. The gunman used both a small calibre rifle and a larger calibre hunting weapon.
During initial questioning by police, the suspect admitted that he had carried out the shootings. The head of the investigation, Mika Ihaksinen of the National Bureau of Investigation (NBI), said on Saturday afternoon that it was too early to speculate about the gunman's possible motive.
"It looks like he shot people on the street at random," said Ihaksinen.
Police say that there was only a single shooter, but it is possible that there may have been other people involved in the incident.
The suspect was captured by police about two kilometres from the scene. He was not armed when found.
President Sauli Niinistö expressed his condolences to the victims and their families on Saturday morning.
Interior Minister Päivi Räsänen praised police for swiftly and successfully dealing with the incident. She added that she believes that the shooting will give even more food for thought to a working group that is currently reviewing reforms to the nation's gun laws.