At 02.16 an alarm was received about an explosion in Gröndal in southern Stockholm.
"We received lots of calls that some kind of detonation had broken out in a stairwell," says Ola Österling, spokesperson for the Stockholm police.
When the police arrived at the scene, it was found that a detonation had blown up a gate whose glass panes had been shattered. There was also damage inside the stairwell.
No person has been physically harmed.
Extensive police operation
The police have made cordons and are conducting an extensive police operation in the area and the surrounding area.
"A large number of police officers are working on the investigation. Forensic scientists and the bomb squad are in place," says Ola Österling.
Police analysts go through material from film cameras set up around southern Stockholm to gather information about the movements of the last few hours.
The police are knocking on doors and talking to people to find witness statements to the incident.
"Large number of personal checks"
At 04 a.m., no one had been arrested for the crime.
"But we have carried out a large number of personal checks during the night to see if anyone can be connected to this incident," says Ola Östling.
Whether the explosion is connected to the wave of violence over the past six months is likely, he says, but stresses that it is "still too early to comment on it".
"We have a number of police officers working around the clock to map the violent events that are going on. But where exactly this blast ends up on that map, we can't say right now.
The crime is classified as public destruction.