(Image Credit: Mary Schwalm/AP Photo)

Officials at Australia's Taronga Western Plains Zoo are running a battery of tests to determine why four white rhinos at the zoo suddenly died.

According to the zoo's website, the rhinos, named Izizi, Aluka, Intombi and Amira, "died suddenly after showing some neurologic abnormalities such as stumbling."

The zoo has a veterinary team working around the clock, consulting with numerous rhinoceros specialists, virologists, and pathologists to determine the cause of death. According to the zoo's website, "the investigation has ruled out exposure to toxins, bacterial infection, snake venom and organ failure as causes of death. "

In the meantime, the zoo's three remaining rhinos have been removed from the display and placed in quarantine where they remain under the close watch of the zoo's veterinary team.

The four rhinos belonged to the southern white rhinoceros subspecies, which has an estimated population of 14,538, according to the World Wildlife Fund. The southern white rhinoceros lives primarily in Southern Africa and is viewed as a conservation success, having increased its numbers drastically from an estimated population of less than 100, a century ago.  Nevertheless, the white rhinoceros remains classified as near threatened, according to the WWF.

"Obviously the Rhino keepers and veterinary staff know and care for every individual in the herd, so this has been a huge shock and we're all very sad and supporting each other through this difficult time," said Matt Fuller, the Taronga zoo's general manager, on the website.  "Our focus is on continuing this investigation to pinpoint the cause of the sickness and to care for the remaining animals in the herd."