SPRINGFIELD, Mass. — Preliminary investigations show more than 40 buildings were damaged in a natural gas explosion in Massachusetts that injured 18 people, building inspectors said Saturday.

A strip club was flattened and a day care center was heavily damaged in the massive blast Friday night in Springfield, one of New England’s biggest cities.

Investigators were trying Saturday to figure out what caused the blast that could be heard for miles, left a large hole in the ground where the multistory brick building housing Scores Gentleman’s Club once stood and scattered debris over several blocks.

Officials already had evacuated part of the entertainment district after responding to a gas leak and odor reported about an hour before the explosion. Gas workers venting a gas leak got indications that the building was about to explode and they ducked for cover behind a utility truck — along with firefighters and police officers — just before the blast, said Mark McDonald, president of the New England Gas Workers Association.

Most of the injured were in that group, and the truck that saved their lives was essentially demolished, he said.

“It really is a miracle and it’s an example of our public safety officials, each and every day, putting themselves in harm’s way, taking what could have been considered a very routine call of an odor of gas, but they took the proper precautions,” State Fire Marshal Stephen Coan said. “And thanks to God that they did.”

Officials also marveled how the 5:30 p.m. blast occurred when a day care center next door was closed. The center’s building was heavily damaged.

Lt. Gov. Tim Murray and Springfield Mayor Domenic Sarno were attending a tree-lighting ceremony when the explosion occurred. Sarno said some people mistakenly thought the boom was part of the holiday event.

The explosion blew out windows in a three-block radius, leaving at least three buildings irreparably damaged and causing emergency workers to evacuate a six-story apartment building that was buckling, police said. Pieces of broken glass littered streets and sidewalks. It was unclear how many residents had been evacuated. A shelter was set up at a school, but city officials said no one stayed there overnight.

Omar Fermin, manager of the Punta Cana Restaurant two blocks from the explosion site, found the floor-to-ceiling windows blasted out when he came to check on the property Saturday morning.

“It looks like an earthquake hit,” said Fermin, a native of the Dominican Republic. “I’ve never seen anything like it.”

He said he was waiting for someone to come and assess the damage. He worried the restaurant would remain closed for weeks while the owner seeks to replace the massive custom-made windows.

Authorities cordoned off the center of the explosion Saturday as building inspectors worked to identify unsafe structures. Anxious residents gathered at the perimeter, waiting for permission to visit their buildings.

Preliminary reports show the blast damaged 42 buildings housing 115 residential units, said Thomas Walsh, spokesman for the mayor.

Three buildings were immediately condemned, and 24 others require additional inspections by structural engineers to determine whether they are safe, Walsh said.

Dogs trapped in abandoned buildings barked loudly Saturday as building inspectors fanned out across the area. One inspector said he wished he could get a ladder and permission to retrieve a dog that was barking in the upper floor of one building sealed off from residents.

Authorities are opening an animal shelter for pets affected by the explosion, Walsh said.

Coan, the fire marshal, said his office is investigating the cause of the blast and its possible origin. The state’s Department of Public Utilities was also investigating.

Sheila Doiron, a spokeswoman for Columbia Gas of Massachusetts, said the company will continue to monitor for any potential leaks within several blocks of the blast site. So far, she said, the company hadn’t yet found any measurable readings.

The utility will keep at least 30 workers at the scene, along with a so-called sniffing car mounted with sophisticated gadgets capable of detecting low levels of gas leaks, Walsh said.

Doiron said the company also didn’t find in its records any gas odor calls to the area where the strip club was located.

Authorities also deployed police officers throughout the area to prevent looting, Walsh said.

The victims were taken to two hospitals in the city. Those injured were nine firefighters, two police officers, four Columbia Gas workers, two civilians and another city employee.

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.