Mourners have gathered outside a school in southern Finland a day after a 12-year-old boy was killed and two girls of the same age were seriously wounded in a shooting.

The young suspect, who attended the school, was apprehended less than an hour later.

The attack shocked the Nordic nation, where Finnish blue-and-white flags were hoisted at half-mast and scores of people including parents, teachers and fellow students laid flowers and lit candles in the snowy landscape.

Police said one of the wounded girls has a dual Finland-Kosovo citizenship. The suspect and the victims were all classmates.

Police tape at the scene of the shooting in Finland
Police tape at the scene of the shooting in Finland (Jussi Nukari/Lehtikuva via AP)

On Tuesday, heavily armed police cordoned off the Viertola school, an 800-student secondary school in the city of Vantaa, just outside Helsinki.

Police said the boy died instantly after being shot.

The suspect was detained in the Helsinki area less than an hour after the shooting, with a handgun in his possession. The gun was licensed to a relative of the suspect who was not immediately identified.

Police said he admitted to the shooting in an initial police hearing, but there was no immediate word of the motive.

Candles and flowers left at the school
Candles and flowers left at the school (Jussi Nukari/Lehtikuva via AP)

The minimum age of criminal liability in Finland is 15 years, which means the suspect cannot be formally arrested. A suspect younger than 15 can only be questioned by the police before they are handed over to child welfare authorities.

Finns often complain their country is portrayed as a gloomy northern outpost of Europe, where long, dark winters drive people to binge drinking, suicide or random outbursts of violence.

Finland has witnessed two major deadly school shootings in 2007 and 2008. In their wake, the country tightened its gun laws, raising the minimum age for firearms ownership and giving police greater powers to perform background checks on individuals applying for a gun licence.

The nation of 5.6 million has more than 1.5 million licensed firearms, and about 430,000 licence holders, according to the Finnish Interior Ministry. Hunting and gun-ownership are deeply rooted traditions in the sparsely populated country.