A condominium exploded while under construction Friday, injuring eight people, including five construction workers. A resident and two firefighters were also injured.

Three of the injured workers were taken away in MedFlight helicopters after the explosion and fire at The Village at River's Edge, in Norfolk, about 20 miles southwest of Boston. Two were taken to hospitals by ambulance from the complex, a new housing development for people over 55.

Norfolk Fire Chief Coleman Bushnell said the two firefighters were treated for minor injuries and released.

Town Administrator Jack Hathaway said the explosion occurred in one side of a duplex condominium where construction was being finished. A woman who lived on the other side of the duplex — which had already been completed — managed to escape. Bushnell said she was shaken up from the blast was taken to a hospital.

Hathaway said one construction worker was trapped in the rubble, in the basement of the building, for about an hour before rescue workers were able to get him out while an active fire was ongoing.

Bushnell said that worker suffered serious injuries, including burns, but did not have details.

No names have been released, Bushnell said.

Hathaway said he felt the blast a half-mile away, while people in the neighboring town of Franklin reported hearing the blast two miles away.

"Right now, three-quarters of the house just doesn't exist," Hathaway said.

State Fire Marshall Stephen Coan said it was too early to know the cause.

Al Rao, who works for Prudential Page Realty, the marketing agent for the complex, said the workers were in the duplex working on the heating and air-conditioning unit. Rao said the units are serviced by underground propane tanks.

"Our main concern is the health and well-being of the construction workers who were injured," Rao said.

Rao said construction on the "active adult community" began in 2007. Forty-two duplex units have been completed. A total of 136 units are planned.

Representatives of DiPlacido Development Corp., the project's builders, could not immediately be reached for comment. Messages were left at their office.

Lorelei Margeson, 66, who lives on a neighboring street, said she heard a huge explosion and her house shook.

"The volume and intensity of it, you get frightened. I've never heard anything like that before. It felt like it was right in your backyard," she said.