SOUTH PORTLAND – A supermarket reopened Tuesday morning after a scare in which more than 50 customers and employees were checked out at hospitals after suffering burning throats, difficulty breathing and nausea.

Officials believe a problem with the heating system was to blame for the symptoms that led to the evacuation of the Shaw’s Supermarket at Mill Creek at about 6:45 p.m. Monday.

“It was not an intentional act. It was an equipment malfunction,” South Portland Fire Lt. Robb Couture said Tuesday.

The manager ordered the evacuation after customers and workers began coughing and covering their mouths with shirts or handkerchiefs.

Twenty-one ambulances from as far away as Freeport and Gray assisted in bringing the workers and customers to three local hospitals. Most of the people who fell ill were feeling better within an hour, officials said.

Investigators were looking at the heating system in the recently renovated store as the possible culprit.

After the building was cleared, workers turned on the heating system and it produced a metallic-type odor, Couture said. Workers noted that part of the ductwork had come loose, and the problem was fixed, he said.

The Northern New England Poison Control Center initially suspected Freon. But workers tested the refrigeration system and there were no leaks, Couture said.

“It could be Freon. It could be another nuisance substance. It could be something that we never know what it really was,” said Dr. Tamas Peredy, a medical toxicologist who was consulting with medical workers on the incident.

Fire officials were unable to detect any foreign substances using sensitive monitoring equipment that can detect everything from carbon monoxide to a chemical or biological attack, Couture said.

The incident was a good test for the city’s emergency response plan adopted after the terrorist attacks on Sept. 11, 2001, he said. “We were able to triage 43 patients in under an hour, and get them to the hospital,” he said.

All told, more than 50 people were either transported to hospitals by ambulances or went to hospitals themselves to get checked out.

Shaw’s was working with local emergency workers to determine the source of the problem. Spokeswoman Judy Chong declined further comment, saying she would let the fire department answer questions about the incident.