Police said they had found several containers of butane gas as well as military munitions in the apartment, but did not elaborate. Authorities initially thought as many as 25 people might have been injured in the explosion just before 9 a.m. but later lowered the number to 15.
Debris from the blast flew up to 60 meters (200 feet) and some people were seriously hurt, police spokesman Uwe Becker told German broadcaster n-tv. Others were evacuated from the building, which has 60 units.
The building is on the edge of Blankenburg, a town of about 20,000 people in the Harz mountains, west of Berlin. About 100 children at a nearby daycare center were taken to other facilities.