SPRINGFIELD — School officials say it may not have been carbon monoxide that sickened 30 children and forced an evacuation of a southern Minnesota school.

Investigators initially suspected a carbon monoxide leak caused nausea and headaches at Springfield Public School Thursday. Thirty children were taken to Mayo Clinic in Springfield, where they were treated and later released to their parents.

Superintendent Keith Kotke said that professional readings taken inside the school so far have been negative for carbon monoxide.

Classes are canceled at the K-12 school where more testing is expected.