The BBC investigates Zhang Zhidong, also known as “Brother Wang”, a Chinese national described by US law enforcement as one of the most significant drug traffickers in the world.

He is now awaiting trial in the US and has pleaded not guilty. He is accused of building a criminal empire connecting China's chemical factories to the Mexican cartels flooding the US with fentanyl.

The BBC investigates his rise and fall, speaking to his former friend, and to cartel members who knew him as “number one” and the "king of fentanyl”.