SPRINGFIELD, Mass. — 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, the state fire marshal announced Sunday.

Friday night’s natural gas blast in one of New England’s largest cities 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 in Springfield.

The blast flattened a strip club, heavily damaged a day care center and scattered debris over several blocks.

Some officials called it a miracle that no one was killed. The cause of the spark that ignited the explosion is still unknown.

The gas company planned a news conference for later Sunday afternoon. A message left for a company spokeswoman wasn’t immediately returned.

Most of the injured were part of a group of gas workers, firefighters and police officers who ducked for cover behind a utility truck just before the blast. Part of the neighborhood already had been evacuated because of reports of a gas leak and odor.

Preliminary reports showed 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.

Springfield, which is 90 miles west of Boston and has about 150,000 residents, is the largest city in western Massachusetts. It’s known as the home of the Basketball Hall of Fame, which is not near the blast site.

The city has been rebuilding from damage caused by a tornado in June 2011.