(CNN) -- Barely an hour after zookeepers heard the mother panda honk in distress, veterinarians pronounced her cub dead Sunday morning.
Staff at the Smithsonian's National Zoo in Washington said they were heartbroken by the newborn giant panda cub's death, just six days after officials trumpeted its birth.
"The keepers, the veterinarians, the volunteers were watching this cub and this birth with great joy (and) with great passion," National Zoo Director Dennis Kelly told reporters Sunday. "The loss, there's no other word for it, it's just devastating."
There were no outward signs of trauma or infection, the zoo said, and it was not immediately clear what caused the cub's death.
After the mother panda's distress call Sunday, a veterinarian examined the cub and observed that it was not breathing and had no heartbeat, said Suzan Murray, the zoo's chief veterinarian. CPR and other life-saving measures were not successful, officials said. Veterinarians pronounced the panda dead at 10:28 a.m.
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 just before 9 a.m., Kelly said.
Investigators will perform a necropsy to determine what caused the cub's death, Murray said.
"We're all very anxious to know what happened, but we won't know that for a little while yet," she said.
The cub, conceived through artificial insemination, 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.
The zoo's panda exhibit will be closed "until we're confident that Mei Xiang is safe and healthy," Kelly said.
Last week, zoo officials had predicted that attendance would increase by a half-million people this year because of the baby panda.
Zoo visitors said they were sad to learn of the cub's death.
Lisa Carlson of Fort Meade, Maryland, first heard about it on the morning news, and was preparing to explain it to her panda-loving, 5-year-old daughter Amber during their family visit to the zoo Sunday.
"We're going to make it a point to buy as (many) panda things as we possibly can today," she said.
In July, officials said a week-old giant panda cub died at Tokyo's Ueno Zoo after its mother's milk accidentally entered its airway.
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 Stacey Samuel, Mike M. Ahlers and Gregory Clary and journalist Martin Rand III contributed to this report.