WASHINGTON (Reuters) - The giant panda cub that died at the National Zoo in Washington last month succumbed to liver disease caused by inadequately formed lungs, officials said on Thursday, releasing findings they hope will help them learn more about breeding the endangered black and white bears. The cub, born on September 16, died when it was six days old in an agonizing blow to wildlife conservation efforts. Veterinarian Suzan Murray of the National Zoo said officials were unsure why the cub's lungs failed to form properly, but said the scientific community hopes to learn more about the causes of death in giant panda cubs. Murray said the cub's mother, Mei Xiang, who gave birth after years of failed efforts at conception, at age 14 could be nearing the end of her cub-bearing years. But Murray did not write off the possibility Mei Xiang could again give birth. "We're hopeful that she would be able to again in the future," she said at a news conference. The cub was never given a name, in line with a Chinese tradition that pandas are not named for 100 days. The arrival was cause for celebration among zoo officials and wildlife conservationists around the world given the daunting odds for the endangered species reproducing in captivity. Fewer than 1,600 giant pandas are known to exist in the wild in China, and about 300 live in zoos and wildlife centers around the world. Breeding is a critical challenge. One in five cubs born in captivity die in their first year of life, the National Zoo said on its website. The cub's death on September 23 was discovered after panda keepers and zoo volunteers heard a distress sound from Mei Xiang. The zoo's staff failed to revive the cub with lifesaving measures, including CPR. Since then, zoo officials have been closely monitoring Mei Xiang, who spent a couple of weeks cradling a rubber toy as if it were a cub. Don Moore, an animal behaviorist at the National Zoo, said the mother had recently set aside the toy and her behavior was slowly returning to normal. He said she was still down about 20 pounds (9 kilograms) from her usual 240 pound (109 kg) weight. He said the zoo had received a flood of support for Mei Xiang. "We did get a lot of cards and letters and even stuffed animals," he said. Writing by Paul Thomasch and Dan Burns; Editing by Vicki Allen