Panda cub dies at Smithsonian's National Zoo September 23, 2012 -- Updated 1640 GMT (0040 HKT) NEW: "We are brokenhearted," the zoo's website says It is not immediately clear what caused the cub's death Zookeepers and volunteers heard "a distress vocalization" from the cub's mother Washington (CNN) -- A giant panda cub born a week ago at the Smithsonian's National Zoo has died, the zoo said Sunday. There were no outward signs of trauma or infection and it was not immediately clear what caused the cub's death, the zoo said. "We are brokenhearted to share that we have lost our little giant panda cub," a post on the zoo's panda habitat website said. Veterinarians examined the cub Sunday morning after zookeepers and volunteers heard a "distress vocalization" from the cub's mother, 14-year-old giant panda Mei Xiang. The cub did not respond to CPR and other life-saving measures, the zoo said. The cub, which the zoo had not yet named, was Mei Xiang's second in seven years with 15-year-old Tian Tian, the zoo's male giant panda. In July, a week-old giant panda cub died at Tokyo's Ueno Zoo after its mother's milk accidentally entered its airway, officials said. The giant panda is one of the most endangered species in the world. There are estimated to be only 1,900 still in existence. CNN's Gregory Clary and journalist Martin Rand III contributed to this report.