The six-and-a-half-day-old cub was found to have unusual fluid in its abdomen and liver irregularities, chief veterinarian Suzan Murray said at a press conference on Monday at the Washington, D.C. zoological park.
It was too early to tell if the abnormalities found in preliminary results of the necropsy performed on the cub contributed to its death.
Lab tests on tissue samples should be completed in the next week and could provide further insights. Suffocation was ruled out after the cub's heart and lungs appeared normal.
"It's a hard time for us," Zoo Director Dennis Kelly said in this morning's announcement. But based on preliminary information, he said, "We know of nothing that we would do differently at this point."
The cub's mother, Mei Xiang, is returning to her normal routine, eating bamboo and interacting with keepers, though is still cradling a toy in her den, a sign to zoo experts that she's yet to transition away from a mothering role.
(Also see "How Do Giant Pandas Survive on Bamboo?")
Panda Cubs Especially Vulnerable
Panda cubs are hard to breed in captivity, given the mother's narrow window of a once-a-year-ovulation cycle. And the newborns often do not survive.
Weighing about four ounces at birth—and roughly the size of a stick of butter—the cubs are especially vulnerable in the face of the mother's 200-pound (90-kilogram) frame. In 2006, a giant panda in China accidentally crushed her newborn as it nursed.
Despite this death, there are captive cubs who survive and the species itself is growing steadily in captivity. Zoo Atlanta and the San Diego Zoo have successfully bred at least eight panda cubs over the last 13 years.
Panda Mountain, a reserve in Wolong Province, China, has also seen success.
"Scientists and veterinarians have made very significant progress with giant panda captive-breeding programs in the last 25 years," said Panda Mountain president Marc Brody.