By Joe Schulz | Wisconsin Public Radio

A mechanical issue in a northern Wisconsin school building caused nearly 40 illnesses Thursday and forced the district to cancel classes and after-school activities.

The Forest County Sheriff’s Office said hazardous materials teams determined an issue with a furnace in the Crandon School Building created elevated levels of carbon monoxide inside the school. 

“Classes and school events for the Crandon School District have been canceled for today,” the sheriff’s office said in a statement Friday. “The School District reports it is making an active effort to make the school safe and rectify the situation to open campus for classes.”

The school was evacuated around 1 p.m. on Thursday after students and staff from different parts of the building reported mysterious illnesses. According to officials, their symptoms included lightheadedness, tingling or numbness in extremities, shortness of breath, slurred speech and rapid heartbeat. 

The students and staff were transported to area hospitals as first responders searched the building.

The sheriff’s office said 39 people received medical attention in relation to the incident. Two of those people had elevated carbon monoxide levels. 

Officials also said at least two emergency medical services workers became ill during the search of the school.

An initial investigation by the local fire department and sheriff’s office did not locate any harmful gasses in the Crandon School Building, nor did K-9 units discover any illegal substances in the building. The sheriff’s office says hazardous materials teams were then requested to isolate and test the building.

In a message to families Thursday, Crandon Superintendent Kirby Schultz said the decision to cancel school and activities that afternoon and all day Friday was made to ensure the safety of the school community.

“We understand the inconvenience and concern this may cause and assure you that we are working diligently to resolve this situation as quickly and safely as possible,” Schultz wrote. “The health and safety of our students and staff are our utmost concern, and we are committed to keeping you informed as we learn more.”

This story was produced by Wisconsin Public Radio and is being republished by permission. See the original story here.