NINGBO, China: A septic tank was the epicenter of a massive explosion that rocked one of China’s largest port cities over the weekend, killing two people and injuring at least 19, authorities have said. The blast tore through a crumbling light-industrial area adjacent to crowded residential towers in the city of Ningbo, just south of Shanghai, and was so powerful that it was heard several kilometers (miles) away. The explosion at the epicenter shattered windows in nearby apartments, mangled cars and reduced small buildings to rubble, though locals said the structures were already in poor shape and slated for demolition. Public security officials “have confirmed that the blast’s epicenter is a septic tank in an empty field,” the local government said in a statement late Sunday. Methane and hydrogen sulphide—both highly flammable gases—can build up in septic tanks. Officials have ruled out the possibility of a gas explosion or of the blast being intentionally set off by someone. The statement said in addition to the two fatalities, two people are missing after the explosion. Four people were severely injured and 15 others had minor wounds.