A group of 40 kindergartners and 2 adults from Lincoln Elementary were taken to hospitals Friday after having difficulty breathing while swimming in pool learning about water safety.

The pool was at a nursing home run by North Central Health Care in Nassau.

Most of the kindergartners were released within two hours, Mary Ellen Marnholtz, representative for the Wausau School District in northern Wisconsin said, according to the Associated Press. Two girls were kept overnight and released Saturday. Only one boy remained in medical care at St. Joseph’s Hospital in Marshfield, his state being upgraded from critical to fair condition Saturday, in the evening. He remained in fair condition last night, a nursing supervising at the hospital said.

Officials at the Marathon County Health Department and North Central Health Care said Saturday a noxious gas, possibly chlorine, caused the emergency, but they are still unsure of where it came from. It might have been a pump malfunction.

“We feel terrible sorry that these children had this happen at our facility,” said Gary Bezucha, Chief Executive Officer of North Central Health Care, according to local media sources. He added that he was looking for clear answers to prevent something like this from happening again.

The pool will remain closed until further investigation.