Officials said that 54 rays died on Friday in a popular exhibit at Brookfield Zoo, located in the Chicago suburb of Brookfield, Illinois, when a malfunction caused oxygen levels in the tank to drop too low.
The incident killed four southern stingrays and 50 cownose rays, which were in the Stingray Bay habitat, a shallow pool that allows visitors to touch the animals.
The zoo released a statement Sunday that said it was working to determine the cause of the problem. The statement also said the seasonal exhibit, which was originally opened in 2007, would remain closed through the summer.
After the incident occurred, a system alerted crews, who were immediately on the scene and tried to help the animals. Ultimately all of the rays died.
"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.
All of the rays were born in captivity, the Chicago Tribune reported.
In 2008, 19 stingrays died at the Brookfield Zoo when a heater unit malfunctioned, making temperatures rise by 10 degrees. The incident did not kill all the animals in the exhibit; 14 rays, eights sharks, and three horseshoe crabs were able to survive.