The sudden illness of dozens of students at a southern Minnesota school on Feb. 6 was not caused by carbon monoxide, officials say. Instead, it was a likely case of mass psychogenic illness — when people in a group start to feel sick at the same time.

“The facts fit the pattern we see in mass psychogenic illnesses,” said Doug Schultz, spokesman for the Minnesota Department of Health.

After 10 students got sick during a choir rehearsal, Springfield Public School was evacuated and 30 students were taken to the hospital. Responders thought a carbon monoxide leak might be to blame.

But all air and blood tests were negative for carbon monoxide, Schultz said. The students who reported symptoms recovered in a few hours. Their complaints — headaches and nausea, mostly — were subjective and couldn’t be tested.

Such illness is “psychological in origin,” Schultz said, “but it is no less real. It’s not like the kids were faking it.”