BANGOR – No one was seriously injured in a Tuesday morning fire at Shaw’s Supermarket on Main Street that caused millions of dollars in damage and forced the evacuation of more than 100 store customers and employees.

All of the store’s inventory was ruined as a result of the 10 a.m. fire that began in a mechanical room at the rear of the building, according to State Fire Marshal Stu Jacobs, who estimated the cost of the damage would run into the millions of dollars. The cause of the fire remains undetermined, he said.

The store “sustained damage resulting from a construction fire in the store’s electrical room,” according to a statement released by Shaw’s Supermarkets Inc., based in West Bridgewater, Mass. Plans for reopening will be made on a day-to-day basis, but the store will be closed all day Wednesday, the statement said.

The fire was contained and extinguished within 40 minutes, but smoke spread throughout the rest of the building. The sprinkler system remained off, having been deactivated during the current construction of an addition to the store, according to Bangor Assistant Fire Chief Vance Tripp.

“The damage extends all the way through the store,” he said. “There was a lot of heat.”

Firefighters had difficulty locating the source of the fire due to heavy smoke, and were forced to use a forklift to break down a rear bay door to gain entry, Tripp said.

Two firefighters were treated, then released from the hospital for minor burns on their necks and wrists, Tripp said.

“When you get into a really hot area like that, you’re looking at not just heat problems but hot steam burns,” he said.

Openings were made in the building’s roof to allow heat and gas to escape, Tripp said, and structural damage is limited.

Freon, nitrogen and oxygen stored in the building could have proved disastrous had it ignited, Tripp said.

“The potential was there to be very, very dangerous,” he said. “[The building] would have sustained a lot more loss.”

The storage and use of such chemicals is common in buildings under construction, he said.

Langford and Low Inc. of Portland is currently performing construction work on the property as part of an expansion of the building. The Associated Press reported welders may have been working at the rear of the building at the time the fire broke out. A representative from the company was not available for comment Tuesday evening.

Pat Rowland of Bangor waited in her car in the Shaw’s parking lot to resume shopping Tuesday morning after being evacuated. Customers were asked to bring their grocery carts to the front of the store and leave the building after the store’s emergency lights activated, she said.

“I didn’t get to buy my groceries,” Rowland said. “I thought the store people were very well organized. There was no panic.”

Gray smoke billowed from the vents at the rear of the brick and steel building, leaving a haze over the store’s nearly empty parking lot, where clusters of employees and construction workers waited in the cold.

Representatives from the state departments of health and agriculture will evaluate the store’s damaged inventory this week, according to Shaw’s spokesman Terrence Donilon.

“We’re going to just get rid of anything that’s unusable,” he said.

Customers awaiting prescription orders are advised to call the supermarket’s pharmacy number at 1-866-875-8108 to have refills mailed to them directly. The nearest Shaw’s supermarket with a pharmacy is located in Waterville. The store is working with the Maine Board of Pharmacy to determine when the pharmacy may be reopened, according to the Shaw’s statement.

Fire crews from Bangor, Brewer, Holden, Hampden and Veazie responded to the scene. The investigation was continuing Tuesday night with several firefighters still at the scene.

Shaw’s opened at the Main Street location in 1996. The 10,000-square-foot expansion project currently under way is expected to bring the structure’s total area to 67,000 square feet.

Shaw’s operates more than 180 stores in six New England states. There are more than 20 Shaw’s stores in Maine employing more than 3,000 people.