The BBC has published an investigation into Zhang Zhidong, a 39-year-old Chinese national known in the criminal underworld as "Brother Wang" or the "king of fentanyl", who US authorities accuse of becoming a pivotal link in the supply of the synthetic opioid to the United States. According to the US Department of Justice, Zhang helped establish the chain that moves the chemical ingredients for fentanyl from Chinese factories to laboratories run by Mexico's Sinaloa cartel, one of the world's most powerful criminal organisations. His alleged role matters because fentanyl kills tens of thousands of people each year, mostly in the US, and has become a flashpoint in relations between Washington, Beijing and Mexico City.

Zhang was arrested in Mexico in 2024, escaped, and was recaptured and extradited to the US in 2025, where he has pleaded not guilty and awaits trial; his lawyer declined to comment. A former US Deputy Attorney General, Todd Blanche, described him as one of "the world's most dangerous traffickers" and accused him of running a global enterprise dealing in cocaine, fentanyl and methamphetamine and laundering millions in proceeds. The article notes that fentanyl is around 50 times more potent than heroin, with a lethal dose as small as a few grains of salt, and that President Donald Trump has branded dealers "narco-terrorists" and cited the trade to justify tariffs on China, Mexico and Canada. Cartel members and former colleagues told the BBC that Zhang, a 2010 Spanish graduate of Peking University, first came to Mexico in 2011 to work for a Chinese-owned iron ore mining company, where he was seen as bright, resourceful and adept at building ties with both officials and the underworld.