All 54 stingrays at an Illinois zoo have died after an equipment malfunction caused a sudden drop in their tank’s oxygen levels on Friday, the Chicago Tribune reports.

“It’s just an incredibly unfortunate accident,” Bill Zeigler, the Brookfield Zoo’s senior vice president of animal programs, told the Chicago Sun-Times. “The staff worked extremely hard, even some of our mammal keepers were in the water. Everyone is very bummed out about the whole thing.”

Zoo officials say they managed to return oxygen levels to normal within 20 minutes of an alarm sounding, but keepers were unable to save any rays. From the Brookfield Zoo Facebook page:

Veterinary staff was promptly on the scene to provide medical treatment to the affected stingrays. Additionally, immediate action was taken by animal care staff to rectify the situation and get the levels back to normal. Despite tireless efforts by staff, all the animals, which included four southern stingrays and 50 cownose rays, succumbed.

According to CNN, a different malfunction at the Brookfield Zoo killed more than a dozen stingrays in 2008.

“It was a water temperature increase,” zoo spokesperson Sondra Katzen told the network on Sunday, “and the exhibit temperature increased by about 10 degrees, and we lost about 16 stingrays in that.”

Zoo staff said they are still trying to determine the cause of Friday’s accident.