Investigators haven't been able to determine what caused an explosion at a Chicago-area silicone factory that killed four workers in May. Waukegan Fire Marshal Steve Lanzi tells the Lake County News-Sun that he's confident investigators located the point of origin for the explosion at AB Specialty Silicones and are ruling out intentional causes. He says the investigation was hampered because the workers who might have known what happened were killed in the blast that destroyed the factory. Lanzi says federal agencies and insurance company investigators are continuing separate reviews. Illinois and Lake County officials alleged in a June lawsuit that the explosion released an unknown amount of chemicals, causing air pollution and leaking into storm sewers, which polluted a wetland and Osprey Lake about a mile away.