The explosion took place in a stairwell to an apartment building in Strängnäs. The alarm came in just before half past two during the night until Monday.
– You have heard a loud bang. A police patrol also heard this. What it is that has slammed is impossible to say, says Tobias Sundström, at the police.
The residents of the property were evacuated while waiting for the national bomb shelter, which has now been on site and inspected the stairwell.
– Someone is behind it, we think, but if any person was the target, it is too early to say, says Tobias Sunström, commander of the rescue center Police.
It is unclear how many people were in the property at the time of the incident, but no people were injured.
Hear more in the player above.