Washington's National Zoo is in mourning for the tiny newborn cub of their panda bear Mei Xiang. Her baby was born a week earlier and was the center of attention, especially since pandas are notoriously difficult to breed in captivity. As of now, the death remains a mystery.

Zookeepers heard loud distress calls from Mei Xiang and rushed into action, but it took them an hour to finally get their hands on her cub, and by then the little one was unresponsive. Veterinarians tried to perform CPR, but Mei Xiang's baby was pronounced dead after six minutes.

There were no outward sides of trauma or disease, but a necropsy performed on Sunday could shed some light on the cause. The cub's last minutes of life seemed to be normal -- at 9 a.m. it was making grunting noises, but only 17 minutes later his mother let the zoo staff know that something was dangerously wrong with the cub.

With only 1,600 estimated pandas left in the wild, the death of Mei Xiang's newest baby (a previous cub survived to adulthood) is a huge blow to conservation efforts. Both Mei Xiang and her cub's father arrived in the United States in 2000 as part of a 10-year (now extended to 15 years) agreement with China to help preserve the breed.

Despite the death of her cub, zoo officials say the mother is resting, but the panda sanctuary will be closed indefinitely and there is no way of knowing if Mei Xiang will ever have another cub to replace the one that she lost.

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