There are no reports of personal injuries, says the police's press spokesperson Leif Fransson.
According to the police, it is likely that a hand grenade has exploded.
One has found remains that appear to come from a hand grenade, says Leif Fransson.
An undetonated hand grenade has also been found and it will be neutralized on site by the police's bomb technicians.
The police have cordoned off an area around the apartment building. The residents of the house have been urged not to leave their homes.
The alarm about the explosion came at 20 o'clock on Friday.
Several people have heard the powerful bang. There are also witnesses who have seen people who have run from the scene. If they have any connection to the explosion cannot be said now.
