Hoping to raise money to buy new animals, the zoo sold the carcass of a 1-year-old tiger in November after the animal died of disease, Dang Gia Tung, the zoo's deputy director, told The Associated Press.
Nguyen Quoc Truong paid $7,800, the highest amount offered from six bidders, Tung said. Truong, 43, also bought another dead tiger the zoo auctioned in 2002, he said.
Truong and Nguyen Thuy Mui, 48, were detained Monday after two live tigers were found sedated in the back seat of a car near Hanoi. Police also found four frozen cats in Truong's home that were to be processed into traditional medicines believed to cure a number of ailments, state media reported.
Truong told police he had bought one of the frozen tigers from a Hanoi zoo.
Tung, the zoo official, said he knew that selling the carcasses was a violation of government rules and the Convention on International Trade in Endangered Species of Wild Fauna and Flora.
"We thought we should auction the tigers' bodies to raise funds to enrich the collection of animals at the zoo," he said.
Under the convention, to which Vietnam is a signatory, the bodies of rare wild animals must be donated to museums or cremated, Tung said. Police were investigating the case, and it was unclear whether action would be taken against the zoo.
International wildlife conservationist Sulma Warne in Hanoi said he was pleased to see police cracking down on animal trafficking.
"It's really disturbing to see an institution such as the zoo partaking in such practices," said Warne, who heads up the wildlife trade and monitoring network TRAFFIC in Vietnam, Cambodia and Laos.
The country's tiger population has dramatically declined since the Vietnam War ended in 1975 due to shrinking natural habitats and poaching. About 100 tigers live along Vietnam's borders with Laos and Cambodia, down from thousands before the end of the war, according to the Forestry Department.
