LEWISTON – A public school gymnasium in Poland was cleared for use Friday afternoon after its evacuation the previous night when a dozen people fell ill due to a mysterious contaminant.

Hospital workers found traces of carbon monoxide among the patients, School Union 29 Superintendent Dennis Duquette said.

Private air monitoring companies found nothing to explain the Thursday night incident at Poland Community School on Route 26, where participants at a cheerleading practice suddenly experienced headaches, dizziness and nausea, Poland Fire Chief Mark Bosse said. The school’s heating system also was checked for leaks. None was found.

More events were planned at the gym Friday night, he said.

Although the school was closed Friday, faculty were allowed inside to retrieve paychecks, he said.

The incident Thursday night was reported around 7 p.m. The town’s firefighters used air monitoring devices, but found no traces of carbon monoxide or toxins to explain the outbreak of symptoms among those using the gym. About a dozen girls were taken to local hospitals, where they were treated and released.

Bosse said the mystery remained unsolved and he hoped it wouldn’t recur.

Duquette said some speculated the poor air quality might have been caused by idling vehicles outside the school at a time when dozens of parents might have been waiting to pick up their kids.

Air intake ducts are located near that area, he said. Although there are “no idling” signs in the area, they are not always fully enforced at night.

Duquette said he made the decision to close the school Friday because firefighters left Thursday night scratching their heads after thoroughly examining every inch of the school.

“The unknown, I thought, was just too great,” he said.

A full slate of events is planned for the weekend at the school, said Duquette, who planned to be on hand just in case.