By Ben Nuckols/Associated Press

WASHINGTON (AP) -- The 6-day-old giant panda cub that died at the National Zoo had liver abnormalities and fluid in its abdomen, but zoo officials said Monday that they don't yet know the cause of death.

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.

A necropsy on the cub will be completed within two weeks, and officials expect to have a definitive answer on the cause of death. Panda cubs are about the size of a stick of butter at birth and are susceptible to infections and fatal injuries.

There was no evidence of fluid in the cub's lungs, which would suggest pneumonia, and the cub was not accidentally crushed by its mother, which has happened to other cubs born in captivity. On any given day in the first two weeks of life, pandas born in captivity have a mortality rate of about 18 percent, zoo officials said. Their mortality rate in the wild is unknown, but only a few thousand giant pandas are believed to remain in the wild. Only a few hundred are in captivity.

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.

There was milk in the cub's gastrointestinal tract, but the initial examination could not determine how well it had been nursing, Murray said.

The cub's mother, 14-year-old Mei Xiang, has come out of her den and started eating again and interacting with her keepers, Murray said. The panda slept well Sunday night and has been cradling a plastic toy.

"We think this is her natural mothering instinct," National Zoo director Dennis Kelly said.

Zoo officials said it was too early to know if they would attempt to breed Mei Xiang again next year, but the breeding program at the zoo will continue. Mei Xiang had five consecutive pseudopregnancies before giving birth on Sept. 16. A pseudopregnancy occurs when a panda ovulates but does not conceive.

Mei Xiang had been artificially inseminated with sperm from her male partner, Tian Tian. The pair's only cub, Tai Shan, was born in 2005 and became the zoo's star attraction before he was returned to China in 2010.

Four U.S. zoos have pandas, but the bears at the National Zoo are treated like royalty. The zoo was given its first set of pandas in 1972 as a gift from China to commemorate President Richard Nixon's historic visit to the country.

The zoo's first panda couple, Ling Ling and Hsing Hsing, had five cubs during the 1980s, but none lived more than a few days.

Because Mei Xiang's other cub survived and she appeared to be taking good care of the newborn, zoo officials were cautiously optimistic about the new cub. Kelly said he was not aware of anything that could have been done to improve its chances of survival.

Thousands of people watched Mei Xiang tend to her cub on the zoo's online "pandacam," which also allowed zookeepers to monitor the mother and newborn. On Monday, fans lamented the cub's death in messages posted on Twitter and other social media sites, using the same hashtag used after the panda's birth: "cubwatch." The video feed of Mei Xiang remained online Monday.

The staff was taking the loss especially hard because of the work they'd put in over the past 6 years to produce another cub, Kelly said.

"Every loss is hard," he said. "This one is especially devastating."

Officials: Panda cub had liver abnormalities