A giant panda born in captivity and then released into the wild died after being attacked by an unknown animal, researchers have concluded.

He Sheng, a 3-year-old male panda, was released into the Liziping Nature Reserve in Shimian county, Sichuan province, in March last year.

Experts at the Chengdu Research Base of Giant Panda Breeding became concerned after receiving abnormal signals from the GPS tag on his neck on Sept 27 and launched a search for the animal.

The body was discovered the next day with wounds on its right shoulder, ear and leg, according to a statement by the base on Thursday.

An examination concluded that the panda had been attacked by an unknown animal and the wounds had caused a bacterial infection that resulted in fatal blood poisoning, it said.

The statement came after netizens accused the research base of not telling the public about what happened to He Sheng.

A source at the base who did not want to be identified said the State Forestry Administration had approved training He Sheng to live in the wild. The base reported to the administration shortly after the body was discovered,but it did not inform the public because researchers could not determine the attacker, the source said.

"Some experts thought the attacker was a dog, while others thought the attacker was a wild panda," the source said.

He Sheng was one of the first pandas chosen by the Chengdu research base in2014 for training to live in the wild. At a meeting in June, experts said they believed the animal had a strong ability to adapt to the wild because he could find food and water as well as protect himself from danger.

Training captive pandas for life in the wild is aimed at enlarging the wild giant panda population and protecting the endangered species.According to China's fourth panda census, released in 2015, there are only 1,864wild pandas in the world.

The Wolong National Nature Reserve in Sichuan has released seven panda cubs into the wild since 2006. Five are believed to be faring well judging by GPS tags.The other two have died.

The first was Xiang Xiang, a male released in 2006 at the age of 5. He was found dead a year later. Researchers suspect he may have taken refuge in a tree after fighting with wild pandas, but had fallen.

Xue Xue, the other panda that died, was released in 2014 and only survived about 40 days. Zhang Guiquan, an expert at the Wolong reserve, said it was likely she had died of trauma from being caged for several days before the release.