SYDNEY (AP) — Four rare white rhinoceroses have died in recent weeks at an Australian zoo after displaying mysterious neurological problems such as stumbling.

The Taronga Western Plains Zoo near the New South Wales state city of Dubbo said in a statement Wednesday that it had begun a veterinary investigation to pinpoint the cause of the deaths and is working with rhino specialists in Africa and North America.

The four rhinos — Izizi, Aluka, Intombi and her daughter Amira — began showing signs of neurological problems two weeks ago, an unnamed zoo official told Australian Associated Press.

The first rhino died soon after the symptoms became apparent and the fourth died over the weekend, the official said.

Three other rhinos survived, zoo spokesman Mark Williams said. Williams declined to answer further questions.

The zoo’s general manager, Matt Fuller, said bacterial infections, snake venom, toxins and many types of viruses have been ruled out as possible causes.

Fuller said in a statement that no other species at the zoo had been affected by the illness and the surviving rhinos, which have been placed in quarantine, are healthy.

“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,” Fuller said.

Fuller said results of a microscopic examination of tissue from the rhino carcasses may be available next week, but he added that it could take several weeks to get results from virology cultures.