From the article at:
http://www.knoxnews.com/story/news/2...les/316341001/
Exact toxin unknown
The exact identity of the deadly toxin remains a mystery. Tests couldn't determine the poison.
"It helps to know that most likely an environmental toxic agent caused this," Zoo President and CEO Lisa New said Friday. "But we will never know, we will not be able to determine that toxin."
Tests on both the animals and their building didn't pinpoint the exact cause of death. No poisons or infections were found in blood from the dead reptiles or unaffected animals. Zoo officials also determined within days of the event the reptiles didn't die from food issues, disease, infection or any human or animal predator.
The deaths happened on the night of a violent thunderstorm that swept through East Knoxville but no zoo systems pointed to any electrical failure of lightning strike, New said.
But zoo officials point out that carbon monoxide, carbon dioxide and electrical shock cannot be tested for in animals' bodies after death and so can't be ruled out as potential causes. Knoxville Utilities Board tests done shortly after the deaths didn't find carbon monoxide, carbon dioxide or methane gas in the building.