(CNN) -- The herpetology team arrived at Zoo Knoxville expecting business as usual, but what they found was a baffling sight: dozens of reptiles on the brink of death.
The team immediately began evacuating the reptiles, giving them oxygen and checking for heartbeats with ultrasound. But, for 33 of the 52 reptiles housed in the building, it was too late.
Among those lost were three critically endangered species: the Louisiana pine snake, Catalina Island rattlesnake and Aruba Island rattlesnake.
The zoo has called this "a devastating loss" especially for the herpetologists, who the zoo says "have dedicated their careers to caring for and advocating for these animals."
The team was left with one big question: how did this happen?
What they do know is the deaths were isolated to one building. What they don't know is what happened in that building that cut its reptile population by more than half.
The zoo is conducting necropsies and an investigation of the building to learn more - and what they can do to make sure it doesn't happen to the 19 creatures that remain.