A utility worker responding to reports of a natural gas leak in one of New England’s largest cities punctured a pipe and an unknown spark ignited a massive explosion that injured 18 people and damaged 42 buildings, the state fire marshal announced Sunday.

Friday night’s natural gas blast in Springfield’s entertainment district was caused by “human error,” State Fire Marshal Stephen Coan said at a news conference. He didn’t name the Columbia Gas Go. worker who pierced the high-pressure pipe.

The worker was trying to locate the source of the leak with a metal probe that tests natural gas levels when the probe damaged the underground pipe, Coan said. A flood of gas then built up in a building that housed a strip club, and a spark touched off the blast, officials said.