Paynesville High School in central Minnesota closed early Tuesday after several students fell ill with suspected cases of carbon monoxide poisoning.
The trouble started Monday night when several students at the school felt dizzy or fainted during a holiday concert.
More students got sick or fainted during the school day Tuesday, prompting officials to evacuate the middle and high school buildings by 11 a.m.
Over two days, a total of 19 students were admitted to the local hospital with symptoms similar to carbon monoxide poisoning. Hospital officials say their CO levels were high, but not at the level of poisoning.
It's still not clear what made the students sick.
"It's still being investigated by school officials and the emergency teams they've brought in to determine if something is going on at the school," according to hospital spokesperson DuDonne Andrie.
All of the students have been released from the hospital.