The Chinese Fentanyl King

Zhang Zhidong was, by every account, brilliant. He graduated from China's most elite university and spoke fluent Spanish. He arrived in Mexico in his mid-twenties to work in mining, and quickly learnt to establish relationships with the whoever the top dog was — whether from the local government, or the underworld.

He’s accused of building a criminal empire that made him a critical link between Chinese chemical factories and the cartels flooding the United States with fentanyl. Cartel members in Mexico called him “El Rey”, the king of fentanyl, or “numero uno”, or his alias “Brother Wang”. He is now imprisoned in the US facing drug trafficking charges.

BBC investigates his rise and fall — speaking to a former friend who worked with him in Mexico , to drug cartel members who say they witnessed his influence first-hand, and to the researchers who have spent years untangling the network he allegedly ran.