Mei Xiang's cub, born Sept. 16, died of liver trouble and signs of lung disease, said Chief Veterinarian Suzan Murray. After a full necropsy, scientists found the tiny female cub's lungs hadn't fully developed and likely weren't sending enough oxygen to the liver. The cub was possibly born prematurely.
Zoo scientists are trying to learn more about how common liver and lung defects are in newborn pandas.
"As unfortunate as this was, this baby and studies of this baby post mortem are contributing to our knowledge of panda reproductive science," said Donald Moore, the zoo's associate director for animal sciences.
Zoo keepers have said Mei Xiang took careful care of the cub. It weighed only about 4 ounces when it died. The tiny hairless, helpless creatures can be easily crushed, but the cub had no sign of injury. A small amount of milk in the digestive system suggested she had nursed.
Panda fans keeping tabs online were devastated by the death. The zoo has received an outpouring of cards and letters from around the world and some donations for conservation.
The birth was a surprise because it hadn't been clear whether Mei Xiang was still fertile.
On Wednesday, animal keepers cleared out the bamboo nest she had built after she stopped sleeping in the den.
"She didn't seem to miss it, wasn't upset that it was gone," said panda keeper Marty Dearie.
Mei Xiang's appetite is slowly returning to normal. She stopped eating and stayed in her den for nearly a month to give birth and care for the cub. Now she's eating about 80 percent of her bamboo, fruits, vegetables and biscuits. She's still about 20 pounds under her normal weight of 240.
Despite the death, the pandas still draw many visitors.
Brian and Robin Ballard of Hopkins, Mich., followed news of the recent birth and came to see giant pandas for the first time Thursday.
"It's great to finally see them," said Brian Ballard, a teacher who used to work in a zoo. He remembers the first panda couple sent to Washington in 1972 as a gift to the U.S. after President Richard Nixon's historic visit to China.
"The cultural exchange that took place then, even by today's standards, I think is huge," Ballard said.