The cub, believed to be female, died Sunday morning, less than a week after its birth surprised and delighted zoo officials and visitors. The zoo had all but given up on the panda mother's chances of conceiving.
"Panda keepers and volunteers heard the mother, Mei Xiang, make a distress vocalization at 9:17 a.m. and let the veterinarian staff know immediately," according to a statement released on the Smithsonian National Zoological Park's website.
Staffers were able to retrieve the cub about an hour later. Veterinarians immediately performed CPR, but the cub was unresponsive. The cub appeared to be in good condition, and there was no evidence of trauma, external or internal, which means that the 4-ounce cub was not accidently crushed by her 216-pound mother.
Pandas born in captivity have a mortality rate of about 18 percent in their first two weeks of life, zoo officials said.
While a necropsy on the cub will be completed within two weeks, the fluid in the cub's abdomen was unusual and could have been a symptom of the liver problem, said Suzan Murray, the zoo's chief veterinarian. The liver, about the size of a kidney bean, was harder than usual and discolored, she said.
The cub's 14-year-old mother, Mei Xiang, is slowly returning to her normal routine. She has come out of her den and started eating again and interacting with her keepers. Watchers noticed her cradling a plastic toy Sunday night.
"We believe this is an expression of her natural mothering instinct," the National Zoo's statement read.
The Zoo's PandaCam showing a live stream view of Mei Xiang's den was back up Monday after being turned off Sunday following the cub's death.
The PandaCam allowed zookeepers, fans, and animal lovers all over the world to peek at the cub and it's mother's interaction. Fans have been sending their condolences via Twitter, Facebook, and other social media sites since news broke of the cub's death.
"We're still reeling from the loss of our giant panda cub, and we feel like the whole world is mourning with us. Our staff is anguished, which is to be expected. Every loss is hard but this one is especially devastating," the National Zoo's statement read. "Thank you so much for your outpouring of support."
ABC News and the Associated Press contributed to this report.