Brookfield Zoo employees found the dead fish early Monday. They were among 32 stingrays in a temporary summer exhibit at the facility that also included nurse sharks, white-spotted bamboo sharks and horseshoe crabs.
The 16,000-gallon pool where the rays were kept has a heating and cooling system that keeps the water temperature around 79 degrees since stingrays are highly sensitive to water temperatures, said zoo spokeswoman Sondra Katzen.
The temperature in the pool somehow rose to around 89 degrees by Monday, she said.
"It's pretty devastating," Katzen said. "It's a pretty tragic accident."
Necropsies have been performed, but zoo officials did not immediately have the results.
The other fish in the exhibit appear to be unharmed and zoo employees were monitoring the other animals closely, Katzen said.
The exhibit is owned by San Diego-based Living Exhibits. The exhibit will temporarily remain closed while the company and utility ComEd investigate the malfunction, Katzen said.
The incident was not the first unusual death at the zoo this year. In January, a male giraffe died from strangulation in what officials deemed a "freak accident."