WASHINGTON (AP) -  

A National Zoo veterinarian says a liver problem was to blame for the death last month of a 6-day-old giant panda cub.

Chief veterinarian Suzan Murray told a news conference Thursday that the cub's cause of death was liver necrosis, or the death of liver cells. Murray says the cub's lungs were also underdeveloped and likely didn't provide enough oxygen to the liver.

The zoo also offered a glimpse of panda mother Mei Xiang in her yard Thursday. Her behavior has slowly returned to normal after the cub's death.

The cub, believed to be female, was born Sept. 16. Zoo officials and panda fans were devastated by its death less than a week later. The birth was a surprise because it hadn't been clear whether Mei Xiang was still fertile.