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(NEXSTAR) – Dozens of people have been taken to the hospital after a carbon monoxide leak at a daycare center in Pennsylvania, according to local authorities.

First responders were alerted to the incident on Tuesday morning following reports of an unconscious child at the Happy Learning Smiles Center in Allentown, a spokesperson for the City of Allentown told Nexstar.

Upon arrival, one of the firefighter’s carbon monoxide (CO) monitors alerted the crew to a possible issue. The firefighter located a carbon monoxide/gas meter in the building, which indicated “dangerously high levels” of CO, the spokesperson said.

Fire officials told WPHL there were 25 children and 8 workers inside at the time of the incident. Of those, 28 were transported to local hospitals in the area, the Allentown Fire Department wrote in an update posted to Twitter on Tuesday afternoon.

A triage center was also set up in the center’s parking lot, according to the city.

Investigators ultimately discovered the leak was caused by a malfunctioning heating unit and a blocked venting system. They noted the building did not have carbon monoxide detectors, which will be required for child care facilities under a new state law that has not fully taken effect yet.

The school will be closed while the updates are performed, the owner said.

Capt. John Christopher of the Allentown Fire Department had earlier told local outlet WFMZ that every ambulance in the city had responded to the day care center Tuesday morning. The station added that officials were calling the incident a “level 1 mass casualty” incident, meaning hospitals in the area should prepare to be overwhelmed.

All of the sickened individuals were said to be in stable condition, according to a city spokesperson.

The Associated Press contributed to this report.