STOCKHOLM (Reuters) - An explosion overnight in the southern Swedish town of Helsingborg caused extensive damage to the entrance of a police station, authorities said on Wednesday. Nobody was injured in the explosion, which occurred shortly after midnight local time and also shattered windows in a neighboring building, police said in a statement.  It is unclear what caused the detonation,  police said. Police said they were carrying out checks in the Helsingborg area but had not made any arrests. Gang-related violent crime in southern Sweden has been in the spotlight in recent years with several shootings in the region.  