"The damage we have documented at this time is a broken window. We are trying to find out whether it exploded inside the stairwell or outside the gate," says police spokesperson Mats Eriksson.
The police's national bomb squad has been called in, and a police helicopter and police dogs have also participated in the operation.
We are checking some people in the area, but no one has been arrested.
The alarm came in just before 7 p.m. on Tuesday evening. The police have opened a preliminary investigation into public danger.
