WASHINGTON (CNN) -- The giant panda cub born a week ago at the Smithsonian's National Zoo has died, officials said Sunday, September 23rd.
Veterinarians at the zoo pronounced the cub dead at 10:28 a.m. Sunday, about an hour after volunteers and zookeepers heard its mother honk in distress.
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're all very anxious to know what happened, but we won't know that for a little while yet," said Suzan Murray, the zoo's chief veterinarian.
Zoo officials said they were heartbroken by the news.
"(Staff) were watching this cub and this birth with great joy, but with great passion," National Zoo Director Dennis Kelly told reporters. "The loss, there's no other word for it, it's just devastating."
A veterinarian examining the cub Sunday immediately observed that it was not breathing and had no heartbeat, Murray said. CPR and other life-saving measures were not successful, officials said.
The cub, which zoo officials said was the size of a stick of butter, had not yet been named.
Its last recorded sound was a grunt around 9 a.m., Kelly said.
Investigators will perform a necropsy, Murray said.
The cub was 14-year-old mother Mei Xiang's second in seven years with 15-year-old Tian Tian, the zoo's male giant panda.
Mei Xiang appeared to be calm as keepers monitored her Sunday afternoon, Murray said.
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.
"These bears are so critically endangered that every panda cub is important," Kelly said Sunday.
CNN's Gregory Clary and journalist Martin Rand III contributed to this report.