The Gladys Porter Zoo is investigating after a group of stingrays were found dead Friday morning. 

A preliminary assessment shows that an electrical issue failed to trigger an emergency back-up system that provides oxygen, filtration and removal of organic waste in the tanks, the zoo said. 

Veterinary staff are conducting necropsies on the stingrays to determine the exact cause of death.

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 No other tanks in the aquarium were affected.

“We are moving forward quickly to determine what happened, so we can take steps to ensure this horrific incident is not repeated,” Zoo Director Patrick Burchfield said in a statement. “We are also attending to our aquatic staff, who are devastated by the loss of these beautiful creatures. We hope for a rapid resolution of our thorough investigation and a quick recovery from this tragic occurrence.”

The zoo has also brought in an independent electrical contractor to determine the “cascade of issues” that caused the support failure.

The group of stingrays consisted of Atlantic rays, cownose rays, Southern rays and yellow rays.