The incident occurred outdoors on Thursday morning near the centre of Viksjö, according to the newspaper Mitti, which was first to report the incident.
No one has been arrested yet.
"We have carried out a large number of investigative measures and the work continues on a broad front," Prosecutor Oskar Hedström says in a press release.
The municipality has sent a message to guardians in primary schools to inform them and urges them to accompany their children to and from school in the coming days.
There are approximately 10,000 guardians who have received it, Per Rydén, press officer at the municipality, tells TT.
