GOTHENBURG, Sweden—Four people were seriously hurt in an explosion and fire on Tuesday in the Swedish city of Gothenburg, and police are investigating whether an explosive device was placed at the scene. At least 16 were taken to hospital after the early morning blast set ablaze an apartment bloc in a central residential area. Fire fighters pulled people from the building while other residents used tied sheets to lower themselves from balconies as grey smoke billowed out of stairwells and windows. Gothenburg’s Sahlgrenska Hospital said hours later that three people were still in intensive care after inhaling smoke, but the injuries were not thought to be life-threatening. Sweden has faced rising gang crime and gun violence in recent years, with rival groups using explosives and firearms to settle scores. Police have added officers, and laws have been tightened, to try to tackle the gangs. Bombings have become an almost weekly …