Chen said the raids are usually professional, complex operations in which each member of a gang is allotted a specific task, including raising funds, committing the robbery, finding clients for the stolen artifacts, transporting and trafficking the items and finding ways of disposing of the profits.

Wei Yongshun, a senior official at the criminal investigation bureau, said, "Usually, the leader of the gang remains in the background to fund the operation, while a special technical adviser locates the burial site and organizes access to the tomb."

The graves are usually located by the use of modern technology, including electronic divining rods and metal detectors, while homemade explosives are used to gain entry. Other equipment can include walkie-talkies, night-vision goggles, powerful lights, oxygen cylinders and antiviral masks, he added.

According to Chen, the robbers always use assumed names when communicating and only keep the stolen artifacts for as long as they have to, preferring to sell them immediately or smuggle them overseas.

"Wealthy businessmen and even high-ranking officials, including some lawmakers and political advisers, buy items for their own collections, or the artifacts are smuggled to private museums or collectors overseas," he said.

Between March 2015 and July last year, police in 16 cities and counties across five provinces recovered 1,100 stolen items, including 222 classified as Grade 1 or 2 cultural relics.

A typical example occurred in July, when police in the provinces of Shaanxi, Gansu and Shanxi smashed eight criminal gangs and detained 91 people suspected of involvement in 96 cases of tomb raiding and trafficking of cultural relics, according to the Shaanxi Public Security Bureau.