MINNEAPOLIS (WCCO) — School officials in Paynesville released students at 11 a.m. Tuesday from the Paynesville Area Secondary School due to “air quality concerns,” after 27 kids became ill.

School officials said parents were able to pick up their students, and that buses to take students home began loading at 11 a.m. at the school, located about 95 miles west of Minneapolis.

Officials tested the school after nearly a dozen children got sick at a high school concert Monday night. The students complained of symptoms related to carbon monoxide poising, such as nausea, headache and dizziness.

“We had fumes that came into the auditorium,” Paynesville Superintendent Bob Huot said. “Our system showed a spike in our vent system with carbon monoxide.”

Officials said they believe Monday night’s cause was carbon monoxide, but Tuesday morning’s reason for the illness outbreak of more than 20 students is still unknown.

That’s because after conducting tests, doctors found no carbon monoxide in the students’ systems after Tuesday morning’s incident.

A spokesperson for CenterPoint Energy said it had workers on the scene Tuesday. The workers think carbon monoxide was responsible for the incident, and they reportedly found and tagged two appliances that might possibly have caused students to get sick. One of the tagged appliances was in the kitchen, the other was a water heater. The kitchen appliance was fixed, and the water heater still needs to be repaired.

To be clear, CenterPoint’s assessment of the incident has not been confirmed by school authorities or law enforcement. Fire department officials are continuing to check the building to make sure nothing else is leaking carbon monoxide in the school.

Investigators from the Department of Health and Homeland Security are also still working to find out what happened.

Classes will resume Thursday.