(CNN)Chicago's Brookfield Zoo is trying to figure out why 54 stringrays died Friday. Veterinary staff said there was a drop in oxygen level at the zoo's Stingray Bay habitat, according to a statement posted Sunday on the zoo's Facebook page.
Animal care workers rushed to try to correct the levels, but four southern stingrays and 50 cownose stingrays were lost. All of the stingrays that died had been born in captivity.
"We are devastated by the tragic loss of these animals," said Bill Zeigler, senior vice president of animal programs for the Chicago Zoological Society, which operates the zoo. "Our staff did everything possible to try and save the animals, but the situation could not be reversed."
Officials are analyzing the life support system to discern what caused the malfunction.
The Chicago Zoological Society has decided not to reopen the stingray exhibit, which has been operating since 2007.