(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 final necropsy determined that lung and liver damage ultimately caused the cub's death. Her lungs were poorly developed and likely caused her to have insufficient oxygen, which would be consistent with the changes in the liver," said a statement on the zoo's website.
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.
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.