One 12-year-old sixth grade pupil died and two others were injured after being shot by a classmate at the Viertola school in Vantaa on Tuesday morning.
The two injured pupils were hospitalised with serious injuries.
Police said they had apprehended a suspect who was also a 12-year-old pupil at the school. The suspected shooter fled the scene on foot and was caught by officers in the northern Helsinki neighbourhood of Siltamäki.
At a press conference on Tuesday afternoon, police said that all of the children involved were Finnish and officials said they had no details to share regarding a possible motive. Police have also said they have not as yet received any indications that the shooting was planned.
The handgun used by the perpetrator was licensed to a close relative, according to the police. The incident is being investigated as murder and attempted murder.
Children under the age of 15 are not remanded into custody in Finland. As a minor, the suspect is not criminally liable.
Politicians soon expressed shock over the shooting, with Prime Minister Petteri Orpo (NCP) and Interior Minister Mari Rantanen (Finns) commenting on the incident.
The shooting occurred at the school's Jokiranta site, which serves pupils in grades 3 to 6. Viertola school, housed in two locations, has some 800 pupils in grades 1-9 and a staff of around 90.
Officers were called to the scene at 9.08am, according to Eastern Uusimaa police. Pupils were kept in their classrooms following the incident, which was said to have taken place in the facility's elementary school.
The facility also includes Swedish immersion groups, special education classes as well as outpatient education.
The Ministry of the Interior has ordered state agencies and institutions to fly the national flag at half-mast on Wednesday, April 3, in remembrance of the Viertola school shooting. The Ministry hopes the whole country will join in flying the flag at half-mast.