WASHINGTON — Washington's National Zoo is set to reveal what caused the death of a 6-day-old giant panda cub.

The zoo will disclose the cause at a news conference Thursday morning.

The cub, believed to be female, was born Sept. 16. Zoo officials and panda fans around the world were devastated by its death less than a week later. The birth was a surprise because it hadn't been clear whether the mother, 14-year-old Mei Xiang, was still fertile.

Panda cubs are born hairless and helpless, about the size of a stick of butter. The mortality rate for females born in captivity is 20 percent in the first year.

A preliminary necropsy revealed that the cub had liver abnormalities and fluid in its abdomen. Zoo officials will disclose the final results on Thursday.

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  • Tai Shan takes a short plunge off the edge of a faux rock during his debut to the press November 29, 2005 at the National Zoo. Tai Shan was sent to China in February, 2010.

  • Mei Xiang and Tai Shan, who was born at 3:41 a.m. July 9, 2005, weighing only a few ounces at birth. This photo was taken in December, 2005.

  • Giant Panda cub Tai Shan cuddles with his mother, Mei Xiang, while they eat melon balls in the morning at the National Zoo's Giant Panda Habitat on August 30, 2006.

  • Giant Panda Tian Tian plays with a box on January 20, 2011 at the National Zoo. The snack is made with apples and pears frozen in apple juice.

  • Giant Panda Mei Xiang eats a breakfast of bamboo at the National Zoo's Giant Panda Habitat on August 30, 2006.

  • Giant Panda Tian Tian enjoys a fruitcicle at the National Zoo on January 20, 2011.

  • The 4th Birthday Of Giant Panda

    Giant panda Tian Tian eats bamboo at the National Zoo on July 9, 2009.

  • Tai Shan officially celebrated his fourth birthday with singing, guests and a massive, three-tiered "veggie-sicle" cake. The frozen masterpiece was made over the course of two weeks by Zoo commissary staff by freezing a combination of water, beets and beet juice while enhancing it with bamboo and fruit. Tai quickly took to the frozen treat, licking at the ice, spotting his furry face with the melting beet juice.

  • Panda cub Tai Shan is examined, weighed and measured on September 19, 2005.

  • The National Zoo's Chief Veterinarian Dr. Suzan Murray, Curator of Primates and Giant Pandas Lisa Stevens and animal keeper Nicole Meese conducted a health exam on Tai Shan when he was a cub in 2005.