Liver failure related to an insufficient supply of oxygen caused the death of the giant panda cub last month at the National Zoo, officials said Thursday.

Chief veterinarian Suzan Murray said a necropsy showed that the tiny cub’s lungs were not fully formed. That impeded the flow of oxygen, leading to liver necrosis, or the death of liver cells.

Murray said it was possible the cub had been born prematurely, but it is difficult to determine exactly when the embryo was formed. The animal’s birth Sept. 16 came as something of a surprise, because a pregnancy had not been confirmed.

Zoo officials said the cub, a female, had fluid in its abdomen and its liver was hard in some places, probably caused by the lack of oxygen. Officials said there was no sign of internal or external trauma.

Since the cub’s death on Sept. 23, its mother, Mei Xiang, has nearly resumed her normal diet of bamboo, fruit, vegetables and biscuits, said Don Moore, associate director of animal care at the National Zoo. She is eating about 80 to 85 percent of what she eats normally when she is not pregnant, he said, and she has shed almost one-tenth of the 240 pounds she weighed before pregnancy. She now weighs 220.

Mei Xiang has stopped cradling a little red conical toy, similar to the Kong toys found at pet stores, that zookeepers gave her after her cub died, and she is venturing out of her den to eat, Moore said.

She had stayed inside her den after the animal was born to nurse, cradle and care for her. As zoo officials provided an update of her condition, Mei Xiang sat in the enclosure nearby, eating a fruitsicle concoction with her back facing onlookers.

Moore described Mei Xiang’s behavior as “almost normal.”

On Wednesday, zookeepers removed the last remnants of bamboo that made up Mei Xiang’s nest inside the den.

“Her hormones have returned to normal levels, as has her behavior,” a news release said. “Mei is choosing to go outside in the mornings. In the afternoons she can usually be found napping on her indoor rockwork. Mei’s appetite has also returned, and she is eating almost all of her bamboo and all of her biscuits and produce.”

No decision has been made about plans for Mei Xiang and the cub’s father, Tian Tian, who is 15 years old. The National Zoo has an agreement with the Chinese to keep both giant pandas until Dec. 5, 2015, although a decision could be made to replace one of the bears if it is in the interest of the pandas, the National Zoo or the species as a whole. Pamela Baker-Masson, a spokeswoman for the zoo, said a decision should be reached later this fall. Both giant pandas can be seen in the zoo’s Panda Habitat, which has reopened to the public.

At 14, Mei Xiang is middle-age for a panda. But, zoo officials said, there is no reason to believe she cannot become pregnant again. Unlike longer-living animals such as gorillas and elephants, which have menopause and stop reproducing later in life, bears typically live into their 20s and can breed into old age, Moore said.

The giant panda cub’s death could contribute to greater knowledge about newborns. Most research into the species begins after the mother and cub emerge from the den months after birth, but very little is known about the initial weeks of life.

It is not clear whether the condition that led to the cub’s death is a common problem for newly born giant pandas, said Murray, the veterinarian.

Zoo officials said they are consulting panda experts and what they called the “human medical community” to learn more about premature births. The death rate for giant pandas within the first year of life is 26 percent for males and 20 for females, but that might be underreported, Moore said.