Up to twenty-five people were injured after an explosion destroyed an apartment building in the western German city of Wuppertal, police said Sunday. Four of the injured are in critical condition, AP reported. Police said the explosion destroyed the several-story building right before Saturday midnight Saturday. The detonation was so severe it destroyed the buildings attic and the top three floors, according to the German news agency (DPA). Firefighters had trouble turning the flames off because parts of the building kept collapsing, however, they rescued four who were severely injured inside the building and sent them to the hospital. According to AP, another 21 people were slightly injured and treated by emergency staff at the scene. Police said Sunday they were still trying to get the fire under control and were investigating the cause of the explosion.