An electrical failure is not believed to be a factor in the death of five stingrays, according to Gladys Porter Zoo Executive Director Patrick Burchfield.

The zoo continues to investigate the death of five stingrays that were found dead Monday at the zoo’s touch exhibit that allows visitors to touch the stingrays.

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The stingrays were fed Sunday afternoon and checked on throughout the night by a watchman and a monitoring system that alerts staff of anything unusual in any of the tanks, Burchfield said.

Nothing out of the ordinary was reported.

It’s really a mystery,” Burchfield said. “We’re sending water samples to laboratories to look for some possible containment that’s beyond our ability to test for.”

Zoo officials said there are a number of possibilities that could have caused the deaths of the stingrays.

The zoo will not replace the touch zone stingrays until they know the cause of deaths of the stingrays, officials added.