The head veterinarian at the National Zoo in Washington says the week-old panda cub who died Sunday had an abnormal liver, but the cause of its death is still unknown.

Doctors say the baby's liver was discolored and hard. The cub also had fluid in its abdomen, another unusual condition for baby pandas.

Condolences from around the world have been pouring in to the zoo. Officials say they are devastated and heartbroken.

The baby's mother, Mei Xiang, cradled a plastic toy as she slept Sunday night. Zookeepers heard her giving a distress call that morning. They found the cub's lifeless body and unsuccessfully tried to restart its tiny heart.

Mei Xiang gave birth last week after five false pregnancies since 2007.

Pandas born in captivity are rare. The National Zoo has had just one cub survive more than a few days since 1972. Others have been born and survived in zoos in Atlanta and San Diego and in the pandas' native China.