Rescue workers gather around a damaged residential block in Shanxi province where three people were killed and six injured in an explosion yesterday morning, Xinhua reported. Four of the 12 homes in the six-storey Datong city building were destroyed, the report said. Gu Shengming , spokesman for the city government, said two of the injured had been discharged from hospital yesterday afternoon, while the other four remained in a stable condition. 'The cause of the accident is not clear and the police are investigating,' Mr Gu said when asked if the blast was caused by illegally stored explosives. Rescuers were still searching for any trapped residents. Shanxi - China's biggest coal producer - witnesses frequent coal mine blasts and accidents involving illegally stored explosives. Last July at least 49 people in Shanxi's Ningwu county were killed by explosives, and in April at least 31 people were killed in a similar tragedy in Yuanping.