KANSAS CITY, Mo. (AP) — Six students and two adults were taken to a hospital for evaluation after a carbon monoxide leak was detected Wednesday at a Kansas City elementary school, officials said.

The eight people taken to hospitals from Longfellow Elementary School suffered nausea and dizziness but none suffered life-threatening issues, Assistant Fire Chief Jimmy Walker said.

Emergency responders went to the school after several students reporting feeling ill. Firefighters found “extremely high” levels of the lethal gas inside the building, Walker said.

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The level of carbon monoxide reached 2,000 parts per million, which was the maximum for monitors used by firefighters at the scene, Walker said.

The cause of the leak is under investigation. Firefighters were ventilating the building but it was unclear when students could return to Longfellow.

School district spokesperson Elle Moxley said the district had contractors check its heating systems last week, including at Longfellow, in anticipation of colder weather. No problems were detected, she said.

Moxley said every child was checked, and the district will work with the fire department before determining where Longfellow students will attend school this week.