Leaders at Zoo Knoxville believe a "toxic agent" caused the deaths of 34 reptiles in one of its reptile buildings in March.
No animals have been kept in that building since then, and the zoo said Friday the building will no longer be used to house animals.
Originally, the zoo said 33 reptiles died overnight in late March. Zoo Knoxville President and CEO Lisa New now connect 34 reptile deaths to the possible toxic agent, including a hatchling that died a week later.
The reptiles died sometime between the hours of 5 p.m. Tuesday, March 21, and 8 a.m. Wednesday, March 22, the zoo said.
Veterinarians at the University of Tennessee College of Vet Medicine determined that the necropsy results, which showed swollen blood vessels and changes in the liver and the heart, were most consistent with a toxic agent.
However, the zoo added, substances like carbon monoxide and carbon dioxide cannot be tested for after death. Because of that, Zoo Knoxville leaders say they may never know for sure what caused the deaths.
Veterinarians tested blood from both the affected reptiles and the surviving ones for multiple toxins, but none were found. No infectious agents were identified either.
When 10News asked if any staff could have been exposed to any dangerous gases, zoo leaders responded that those who found the dead animals quickly ventilated the building and have since showed no signs of illness.
CEO Lisa New also said a storm blew through that night. The test results do not rule out death by electric shock, although the zoo saw no indication of a lightning strike.
New said they don't believe the incident was the result of foul-play.
Zoo staff tested the air quality of the building, and found it to be within normal ranges. They also tested the building's temperature monitoring systems, and it worked reliably during the tests.
The zoo said taking the building out of use is the "safest course of action." The other buildings in the zoo's reptile facility were not impacted, and those buildings are still being used for housing and are open to public viewing.
The zoo is a part of a larger conservation group that will help replenish the lost reptiles at Zoo Knoxville. Officials are currently raising funds for a new reptile facility in the center of the campus, which they hope to have open by the year 2020.