The body of the stricken panda cub rested on a blue blanket in a keepers’ room at the National Zoo. The veterinarian had been trying in vain to revive the animal. But nothing was working. Should she keep going?

The room, filled with stunned keepers and curators, was still. It had been over an hour since the cub was spotted lifeless in the den. Chief giant panda curator Brandie Smith didn’t want the effort to stop, but she knew it was futile.

Should the resuscitation continue?

“No,” Smith said.

People hugged and wept. And for a few moments Sunday morning in the quiet of the panda house, the animal’s body, outwardly perfect, was left on the blanket so the staff could grieve.

Smith and other zoo officials, who recounted the scene, said Monday that a necropsy performed on the cub’s body showed unusual fluid in its abdomen and irregularities in its liver. But it was not clear if that played a role in the animal’s demise.

The zoo’s chief veterinarian, Suzan Murray, said at a news conference that results of lab tests on tissue samples, expected within the next week or so, could help identify the cause of death.

The cub, born Sept. 16, had heart and lungs that appeared normal, suggesting that suffocation was not a factor, Murray said. There was milk in the cub’s digestive tract, a sign that it had been nursing successfully.

Murray said that the cub appears to have been female, although it was so small that lab tests will be needed to confirm that finding. Even without a conclusive ruling, she said, zoo officials have decided to refer to the cub as a “she.”

The cub’s surprise birth came after five failed attempts to impregnate the zoo’s female adult giant panda, Mei Xiang, and experts thought the chance of her having another cub was less than 10 percent.

Zookeepers were overjoyed, and zoo director Dennis Kelly said last week it was a glorious event for the Washington area.

Monday, though, zoo officials were drawn and emotional in recounting details of the cub’s death.

“We were living the promise,” Smith, the panda curator, said at the zoo, her eyes filling with tears. “The promise of all these years to come, and how amazing it was going to be.

“Part of it is . . . the loss of this animal, but it’s also the loss of all that wonder that we were about to be a part of,” she said. “How much fun we were all going to have.”

Smith said the cub appeared to be doing fine early Sunday morning.

“We saw the cub . . . at 8:30,” she said. “It was squirming. It was moving.” She said Mei Xiang was holding the cub and both looked well.

Suddenly, about 45 minutes later, Mei Xiang got up and began honking in “distressed vocalization” for about 30 seconds, and keepers saw that the cub had stopped moving.

An emergency was declared. The cub was removed from the den and rushed to the keepers’ room in the panda house. There, zoo veterinarian Nancy Boedeker began heart massage while staff members, including director Kelly, looked on.

Tense minutes passed.

Smith remembered that the cub was a “beautiful little baby. Absolutely beautiful. Perfectly formed. . . . Black spots were just starting to come out on its eyes. Just a beautiful little baby panda.”

The atmosphere in the room was subdued.

“This has been a dream,” she said. “We’ve all been in this kind of state of suspended disbelief that something so magical [had] happened to us. Everyone just stopped and watched and waited. All feelings, all emotions were suspended for that moment in time. All we could do was hope that the cub would be fine.”

Finally, Boedeker said she believed resuscitation efforts were futile, although she would continue if the curators wanted.

“I said I wanted her to continue because that way we could put off the inevitable,” Smith said. “But at that point, I said ‘No.’ It’s like we were in a bubble. I didn’t want the bubble to pop.”

Once it did, Don Moore, the zoo’s associate director for animal-care sciences, said it was important to let the staff grieve. So the cub’s body was left on the table surrounded by those who were dedicated to taking care of it.

“It was like any death,” Smith said. “We grabbed each other. We hugged each other. We cried.”

Meanwhile, zoo officials said Monday that Mei Xiang appears to be coping well. She slept soundly in her den and has ventured out to eat and drink and interact with her keepers — behaviors she had eschewed at the end of her pregnancy and since the cub was born.

But she is still cradling a toy in her den, much as she cradled her cub during its brief life. Murray and Kelly said the cradling behavior is one sign that Mei Xiang has not transitioned away from the mothering role.

The cub’s sudden death seems to have upset, for the moment, all the plans for a new era of giant pandas at the National Zoo and in the Washington region. Kelly said Monday he had not had a chance to think much about the future.

Giant panda cubs, like many newborns at the zoo, are extremely fragile. A total of six giant panda cubs have died at the zoo, going back to the 1980s. A seventh cub was stillborn. The only cub to survive into maturity has been Tai Shan, who was born to Mei Xiang and Tian Tian in 2005 and was sent to China in 2010.