The National Zoo on Thursday announced that liver problems, likely caused by underdeveloped lungs, are what killed the giant panda cub born there last month.

The liver damage was caused by cell death that appeared to be due to not enough oxygen reaching the liver. Officials also said the lungs were found to be poorly developed, which would have decreased the oxygen flow.

"There is evidence of lung disease, and the lungs were not fully formed," said Dr. Suzan Murray, the zoo's chief veterinarian.

There was also no evidence that the problems were genetic.

(See a photo gallery of the zoo's pandas)

But in a surprise announcement Thursday, spokeswoman Pamela Baker-Masson said the zoo has received more than $10,000 in total donations as part of the "outpouring of support" in the weeks since the 6-day-old panda's death.

"Ten thousand dollars goes a long way around here," she said. "It covers the cost of nutrition, feeding, medicine repairs and upgrades to exhibits, work in the field in China, travel costs -- believe me, we stretch every dollar around here, so it's very meaningful."

The baby female panda was born on Sept. 16, sending a ripple of excitement through Washington and across the nation. She died six days later, before zookeepers had a chance to fully examine her and give her a checkup.

The zoo is also planning to meet with colleagues in China about the possibility of exchanging one or both of the zoo's giant pandas for others in China. In 2011, officials reached an agreement in which the pandas could be exchanged if either panda in the United States "was found unsuitable for breeding," said Baker-Masson. However, she stressed no decision has been made yet about such an exchange.

Dr. Don Moore, the zoo's associate director of animal care sciences, said the mother, Mei Xiang, has resumed normal behavior. Though about 20 pounds lighter, the 220-pound animal is out of the mother's den that zookeepers made for her last month and back in the panda exhibit. Moore said she stopped cuddling a Kong toy several days ago that she had taken up with after her cub's death. Zookeepers removed it Wednesday, as well as cleared out the den.

Moore said infant mortality rates among pandas are 20 percent for females and 25 percent for males. However, he added that those numbers are estimates, as scientists still have much to learn about panda births.