Injections leave the whole body intact and require participation of doctors. Organs can "be extracted in a speedier and more effective way than if the prisoner is shot," says Mark Allison, East Asia researcher at Amnesty International in Hong Kong. "We have gathered strong evidence suggesting the involvement of (Chinese) police, courts and hospitals in the organ trade."

Executions in death vans are recorded on video and audio that is played live to local law enforcement authorities - a measure intended to ensure they are carried out legally.

China's refusal to give outsiders access to the bodies of executed prisoners has added to suspicions about what happens afterward: Corpses are typically driven to a crematorium and burned before relatives or independent witnesses can view them.

Sixty-eight different crimes - more than half non-violent offenses such as tax evasion and drug smuggling - are punishable by death in China. That means the death vans are likely to keep rolling.