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FLEETWOOD, Pa. (AP) — A Pennsylvania school has been shuttered after 12 people were sickened there by carbon monoxide.

Fleetwood Area Schools Superintendent Greg Miller says a community theater group was practicing for a play in the district’s middle school when group members started getting sick around 8:30 p.m. Thursday.

(Photo Credit: CBS Philadelphia)

Twelve people were treated at hospitals and are expected to recover. Authorities eventually determined that carbon monoxide caused the problems, and classes there were canceled Friday as a precaution.

The rest of the districts’ schools were not affected.

Investigators are still trying to determine what caused the carbon monoxide issues.

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