Tigers were suffering from sleeping sickness

Though veterinarians administered antibiotics to some of the zoo's other tigers, nine of them have died since Monday.

Nandankanan housed 56 Royal Bengal tigers, including 32 rare white tigers, before these deaths.

Negligent officials

A five-member team that conducted post-mortems on the dead tigers have blamed Nandankanan officials of negligence.

"The tigers have died of a common parasitical infection called trypanosomysis," one of the team members, Professor AT Rao, told AFP.

"The zoo-keepers should have observed the classical symptoms and started medication a week ago," he said.

Most Indian tigers are found in the wild

Wildlife experts have told the BBC that the drug used to cure trypanosomysis is highly toxic and difficult to administer.

So there is a possibility that it was incorrect dosage which proved fatal.

Zoo conditions

The complete facts will not emerge until the investigation is completed, but the deaths have raised concerns about the conditions in India's zoos.

Some wildlife campaigners say there is a lack of expertise in many zoos because of poor pay, which makes it difficult to attract good staff.

Other experts say lessons should be learned from these deaths about crisis management in zoos, and the need for better strategies to cope with outbreaks of disease.

Fifty years ago, India had about 40,000 tigers in the wild. Now, the population has fallen to about 4000, many of them in national parks and sanctuaries.