The Nghe An environmental police department Friday said the eight tigers, which were taken to the Muong Thuong ecotourism site, died for unknown reasons, the Nghe An newspaper reported.

Their bodies have been frozen for further investigation.

The dead tigers were among 17 Indochinese tigers, each weighing 200-265 kilograms, found being raised in the basements of two families in Nghe An's Yen Thanh District, police said Wednesday.

The two tiger-raising facilities in Yen Thanh District are owned by Ho Thi Thanh, 31, and Nguyen Thi Dinh, 50.

The tigers were confined to separate metal cages after being transported from Laos to Vietnam as cubs, and were kept for months, the two female suspects said.

The tigers were sedated before being taken to the ecotourism site for health checkups. Thanh and Dinh meanwhile are being investigated for the alleged illegal captivity of wildlife.

Indochinese tigers are listed as endangered on the IUCN Red List. Their population is threatened by poaching for illegal wildlife trade, as some people believe their bones could be used to make traditional medicine to treat certain ailments.