Panda Cub Dies at National Zoo WASHINGTON - A giant panda's distress call sent keepers at the National Zoo rushing to the aid of her tiny, lifeless cub on Sunday, and the elation over the cub's birth just a week earlier turned to despair over its death. Zoo officials did not know what killed the cub, which had appeared to be healthy, but a necropsy was performed on Sunday afternoon, said the zoo's chief veterinarian, Dr. Suzan Murray. Zookeepers realized something was wrong on Sunday morning when Mei Xiang, the cub's mother, issued a distressed "honking" noise, officials said. It took an hour to retrieve the cub, and the workers found it unresponsive. Veterinarians tried to revive the cub, performing lifesaving measures including CPR. But they pronounced the cub dead six minutes after retrieving it from the pen. Clearly grief-stricken, zoo officials described the death as a significant blow to the zoo and to international conservation efforts. "These bears are so critically endangered that every cub is important," Dennis Kelly, the zoo's director, said at a news conference on Sunday. This is devastating for all of us here. Conceived by artificial insemination, the cub was born on Sept. 16, delighting zoo staff members because its 14-year-old mother has had five failed pregnancies since 2007. Her chances of conceiving were believed to be less than 10 percent, the zoo said in a statement. The cub's sex was not yet known; veterinarians allow mothers and cubs to bond for a few weeks before examining newborns, said a spokeswoman, Lindsay Renick Meyer. In accordance with Chinese tradition, pandas are not named for 100 days. Zoo officials said the cub was in healthy condition, with no outward sign of trauma or infection. In a video the zoo released on Friday, the cub is shown squirming and heard howling in its mother's arms. The last sign of life was a grunt logged by zookeepers around 9 a.m. on Sunday, Mr. Kelly said. Staff members and volunteers heard Mei Xiang's distress call 17 minutes later. Zoo officials said that they were monitoring Mei Xiang and that she appeared to be calm and was resting. They would not speculate on the likelihood of her becoming pregnant again. Mei Xiang already has one cub, Tai Shan, who was born in 2005 at the National Zoo and now lives at the China Conservation and Research Center for the Giant Panda in the Wolong National Nature Reserve in Sichuan Province. Both of her cubs were fathered by the zoo's 15-year-old male, Tian Tian. Mei Xiang and Tian Tian arrived at the zoo in 2000 as part of a 10-year, $10 million loan agreement with China. A new agreement was signed last year extending the panda breeding program at the National Zoo until 2015. The zoo's first giant panda couple, Ling-Ling and Hsing-Hsing, had five failed pregnancies in the 1980s. Some estimates put the number of giant pandas left at about 1,600. But because they are so difficult to observe in the wild, it is hard to say exactly how many there are, said Pierre Comizzoli, a research biologist with the Smithsonian Conservation Biology Institute. Visitors continued to enjoy the National Zoo under blue skies on Sunday afternoon, some pausing as zoo officials gave a somber news conference near the Asia Trail exhibit, where the pandas are housed. "Maybe we can see the panda babies!" a young girl called hopefully to her family as she skipped through the exhibit. Pandas! Pandas!" a young boy cried excitedly, jumping in circles. "But we can't see them right now," replied another boy as he scanned the empty outdoor pen. Officials said the panda house would be closed indefinitely. Peering into the habitat with her infant son in tow, Mary Gauvin, 28, of Fort Drum, N.Y., said she had learned of the cub's death on Twitter. Having seen the Panda Cam on the National Zoo's Web site, she had planned a visit as part of a family vacation. Her brother, Matthew Amsden, 37, of Los Angeles, said he had seen the giant pandas at the San Diego Zoo. There are 12 giant pandas in captivity in the United States, housed at the zoos in Atlanta, Memphis, San Diego and Washington. Zoo Atlanta and the San Diego Zoo have two cubs each. The youngest cub was born in San Diego in July. Most of the giant pandas in captivity live in China, where scientists are considering reintroducing some of the animals into the wild. Some visitors were surprised to learn of the cub's death when they arrived at the zoo. Murlene Leech, a 46-year-old resident of nearby Falls Church, Va., said she had seen the pandas a few years ago. "It's just really sad," she said. He was just born, and everybody's really excited about it because we're kind of known for our pandas. Pamela Baker-Masson, the zoo's associate director of communications, said the zoo had shared news of the cub's death on its Web site and through social media. As of Sunday afternoon, there was no sign posted yet at the giant panda exhibit to inform visitors of the cub's death. But an earlier sign explaining that the panda house was closed because of the cub's birth remained in place: "SHHH ... Please be quiet and respectful of mother and cub. Emmarie Huetteman reported from Washington, and Michael Schwirtz from New York.