The skulls were first found by a local herdsman named Jiang Caiban on February 25, said local media reports.

Jiang recalled that when he passed the ravine on the day in question, one of his bulls suddenly lost control and ran down the ravine. He followed the bull down and saw a skull in the grass. Under a nearby pine tree, he found more skulls without tops, packed into white sacks

The 66-year-old herdsman was too scared to tell anyone until one month later. His friend Zhang Haoxing, who was told the story, reported it to the town's police bureau.

Local police initially suspected the skulls belonged to monkeys after analyzing their shapes and conducting a preliminary analysis on fur and hair from the skulls.

But Liu Qiufa, a life science professor from Lanzhou University, in the provincial capital, determined the skulls were human after he examined 13 samples on March 31.

A special team of forensic scientists, DNA specialists, and anthropologists sent by the Ministry of Public Security to investigate the case confirmed the judgment on April 4.

The team's investigation showed the skulls were from people of all ages, men and women, old and young, and there were no signs of medical expertise in the decapitations, nor signs of fatal injuries.

Handicrafts made of skull tops, such as a special kind of bowl named "Kapala," were traditionally used in rituals in China's western regions, a report by the Weekend said. "It reflects a different folk custom," a folklore expert Liu Dong was quoted as saying.

"In some western areas such as Gansu, people traditionally believed that tops of skulls resemble goodness and kindness. Nicely decorated handicrafts made from skull tops were kept as a token of remembrance to the dead."

But the report said some people are still selling such handicrafts to make illegal profits, the report said.

An online shop named "Meeting Lablang" was reportedly selling Kapala bowls at prices ranging from 1,100 yuan (US$136) to 1,600 (US$198). Online descriptions said the skull bowls are from Xiahe County in Gansu, about 300 kilometres from where the skulls were found.