Washington (CNN) -- A lack of oxygen due to underdeveloped lungs caused the death of a newborn giant panda at the Smithsonian's National Zoo in Washington, officials said Thursday.
The 4-ounce cub, who had not yet been named, died in September, six days after its birth.
The zoo's chief veterinarian, Dr. Suzan Murray, said the death resulted from lung and liver damage linked with "not enough oxygen getting to the liver."
"This is most likely due to the lungs that were not fully formed, that were impeding the flow, the proper absorption of oxygen, which then led to the death of the liver cells," she said.
The cub, conceived through artificial insemination, was 14-year-old mother Mei's second in seven years with 15-year-old Tian Tian, the zoo's male giant panda.
A pathology report and necropsy determined the cub had a "small amount of milk," in its gastro-intestinal system, suggesting she had nursed with her mother, who was seen Thursday in an outdoor area where officials announced the findings.
Tian Tian was also seen nearby, munching on bamboo and a frozen apple and pear concoction zookeepers provide as a treat to the giant pandas.
From previous births among giant pandas, experts constantly worry about physical trauma for the tiny newborns, which are about the size of a stick of butter. Mei Xiang as of Thursday weighs about 200 pounds, officials said, but noted the necropsy for the cub showed no signs of crushing or other external physical problems.
The death of the cub brought a crushing end to a joyous time at the zoo, where the birth of a new member of the highly endangered species was cause for celebration.
Public interest was also intense. In addition to a camera set up to view mother and cub, zoo officials had predicted the baby panda would push up attendance by a half-million people this year.
A species expert at the zoo, Dr. Don Moore, said the cub's developmental problems were probably not linked to the mother's health or prospects for trying again.
Both pandas are on loan from the People's Republic of China, the native region for the endangered animals. American zoo officials are consulting with their Chinese counterparts about panda reproduction and ways to encourage newborns to thrive in captivity.