SPRINGFIELD, Mass. – Preliminary investigations show 42 buildings were damaged in a natural gas explosion in this city 90 miles west of Boston that injured 18 people, building inspectors said Saturday.

A strip club was flattened and a day care center was heavily damaged in the massive explosion Friday night in Springfield.

No one was killed in the explosion.

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 that housed 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 officials said no one stayed there overnight.

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 Mayor Domenic Sarno.

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

The city was damaged by a tornado in June 2011.