At a press conference on Thursday morning, National Zoo officials announced that the 6-day-old panda cub that died last month passed away because its lungs weren't fully formed, leading to the cub's liver not getting enough oxygen.

Zookeepers began removing panda mother Mei Xiang's nest she had formed for her panda cub on Wednesday, according to NBC Washington. Mei Xiang was reportedly cradling a toy after the cub's death.

Though initially many were worried the mother had accidentally crushed the panda, which often happens in the wild, officials assured reporters that was not the case with this particular cub.