The trainer, Doan Huu Tai, was about to begin painting the cage of the 9-year-old elephant, Ka, Tuoi Tre News reported Wednesday.
Ka grabbed Tai with his trunk, smashed him against the wall of the cage and threw him into a water basin Monday, reports indicated. The zookeeper died later from a brain injury.
The 28-year-old keeper had worked with the 2-ton elephant for at least seven years at the Dai Nam Tourist Center in Thu Dau Mot City north of Ho Chi Minh City.
The smell of the paint Tai brought into the cage may have irritated the animal and made it angry, said Dr. Pham Viet Lam, director of the Saigon Zoological and Botanic Gardens.
He said elephants can be affected when confronted with unfamiliar factors such as color, sound or smell.
Elephant trunks are very sensitive to smell, said Dr. Vu Ngoc Thanh, a biology lecturer at Hanoi National University. He said the smell of the paint may have made the elephant uncomfortable, so he attacked the trainer.