At least four people were seriously injured in a chemical explosion at Dow Chemical Company’s North Andover, Massachusetts, plant on Thursday afternoon, according to authorities. First responders were stll at the scene on Willow Street hours after the explosion was first reported. State Fire Marshal Stephen Coan said that the investigation will begin once the scene is cleared by hazmat crews, and that it appears a chemical reacting to water - trimethylaluminum - caused the explosion. Lawrence General Hospital spokesperson said earlier that four patients suffered critical injuries due to shrapnel and burns from the blast.