GRAND RAPIDS, Mich. (WOOD) – John Ball Zoo officials are investigating after three sharks and 18 stingrays died Thursday night due to an apparent mechanical malfunction, a zoo release said.

Zoo staff found the cownose stingrays and bamboo sharks dead around 7:30 a.m. Friday morning in the stingray exhibit’s lagoon.

It’s believed that Thursday night’s heavy rainstorm caused flooding, which shorted a circuit and turned off one of two pumps in the lagoon, zoo officials said. Additionally, a backup system that was supposed to alert staff in the case of a pump shutdown failed to do so.

Water samples were taken Friday from the lagoon and zoo staff found low oxygen levels in the water. Those samples are being sent to two labs for more testing.

This is a heart-wrenching incident, and we are mourning the loss of these special animals that brought so much joy and amazement to so many people,” said John Ball Zoo CEO Peter D’Arienzo in the release.

“We are currently reaching out to staff and community members who are grieving, and will determine as a family the best way to honor and memorialize these amazing and special animals,” he continued. “We appreciate the continued support of the community as we work through this difficult situation.”

Zoo officials said a new stingray exhibit is being planned for the future.

Rachel Mathews with PETA, People for the Ethical Treatment of Animals, released the following statement on Friday regarding the incident at John Ball Zoo:

“There are few exhibits deadlier than a touch tank, where sensitive aquatic animals are unable to escape a constant onslaught of groping hands that dirty and pollute the water with bacteria and where a single malfunction or mistake could suffocate and poison dozens of animals in one go. We saw it last night at the John Ball Zoo, and we saw it last year when an oxygen malfunction killed 54 cownose rays at the Brookfield Zoo. PETA is calling on the John Ball Zoo to take the only reasonable action here and close down the deadly touch tanks for good.”