SPRINGFIELD, Mass. (AP) — A natural gas explosion that injured 18 people and damaged 42 buildings in Springfield's entertainment district was blamed Sunday on a utility worker who accidentally punctured a high-pressure pipeline while looking for a leak. State Fire Marshal Stephen Coan said the Friday night blast in one of New England's largest cities was caused by "human error." He didn't name the Columbia Gas Co. worker who pierced the pipe while responding to reports of a gas leak.