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.