SPRINGFIELD, Mass. (AP) â¿¿ A natural gas explosion that injured more than 20 people and damaged 42 buildings in Springfield's entertainment district was blamed on a utility worker who accidentally punctured a high-pressure pipeline while looking for a leak. The president of the gas company involved says the employee followed proper procedure and protocol. State Fire Marshal Stephen Coan on Sunday 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. The worker damaged the underground pipe while using a metal tool to locate the source of the leak, Coan said. A flood of gas then built up in a building that housed a strip club, and some kind of spark touched off the blast, officials said. Coan said the employee was following older markings on a sidewalk that indicated the location of the gas line. He appeared to be an appropriate distance from the line, but the markings were incorrect and the worker accidentally punctured the pipe. Columbia's president said the employee followed the correct procedures. "You drive the hole to determine if there is any gas outside," Steve Bryant said. "He stepped over two feet and it turned out to be exactly the amount that the service was offset from the valve, which is a very unusual circumstance." Bryant said the employee had the presence of mind to order an "appropriate" evacuation of the building. Columbia, a subsidiary of public company NiSource Inc., plans to open a claims center at City Hall on Monday for residents and businesses affected by the explosion. Preliminary reports show the blast damaged 42 buildings housing 115 residential units. Three buildings were immediately condemned, and 24 others require additional inspections by structural engineers to determine whether they are safe. The building that housed the Scores Gentleman's Club was destroyed.