National Zoo officials said Monday that their post-death examination of the weekold giant panda cub revealed several abnormalities, although they won't know the cause of death for about two more weeks.

Chief Veterinarian Dr. Suzan Murray said the necropsy showed the liver "felt a little bit hard in places."

"It also did not appear normal; its color was not uniform," Murray said. "This can suggest a liver component to the death, but we won't know until we get the slides back and the pathologists are able to examine that."

She also said veterinarians found "free fluid" in the cub's abdomen. Fluid is common in adults but not in newborns.

"The amount appears moderate to increased for such a small cub," she said.

A preliminary examination also showed the cub was female.

Dennis Kelly, director of the National Zoo, said the cub's mother, Mei Xiang, is starting to return to her normal routine. But its unclear whether she understands what happened.

"Sadly, we have witnessed her cradling [a small toy] most of the night," Kelly said. "This was the same behavior we witnessed right before the cub was born, so we think this is her natural mothering instinct."

The zoo staff, which celebrated the cub's birth just one week ago, is mourning the loss. Zookeepers who last week talked excitedly about getting "glimpses" of the newborn and of hearing its "squawk," stayed out of the public eye Monday.

"There's great sadness amongst our staff, and that's to be expected," Kelly said. "Every loss is hard. This one is especially devastating."

The birth marked Mei's second cub from artificial insemination. After natural mating failed this year, Mei was inseminated in April with semen from her mate, Tian Tian, that had been frozen in 2005. The zoo's adult giant pandas and any cubs they produce are all property of the Chinese government. This pair of adults has been on loan from China since 2000, and will stay until at least 2015, according to an agreement between China and the zoo.

(View photographs of Mei Xiang and Tai Shan, the last giant panda cub born at the zoo)

Giving birth in captivity is rare -- Mei had a less than 10 percent chance of conceiving via artificial insemination, and she has had five false pregnancies since 2007.

The panda house will remain closed until Mei fully returns to her normal routine, which is expected to be in about a week, zoo officials said. However the zoo's panda cam will remain on.

Kelly said officials hope to find a silver lining.

"We want to understand what happened to this cub," he said. "We have to learn from this tragedy, and hopefully we will get a better understanding of giant panda reproduction and cub health as a result."