'Brother Wang was very important. He was number one,' says Enrique, chuckling knowingly. Enrique – not his real name – describes himself as a high-level co-ordinator in Mexico's Sinaloa cartel, one of the world's most powerful criminal organisations. This was reported by Qazaqyia.kz citing BBC News.

On the outskirts of Sinaloa's state capital city, Culiacán, sitting in a parked car where no-one can overhear him, he explains how ingredients to make the deadly drug fentanyl are shipped thousands of miles from Chinese factories to laboratories in Mexico. Members of his cartel credit Brother Wang with establishing this supply chain.

Known in the criminal world as the 'king of fentanyl', Brother Wang is a 39-year-old Chinese national, whose real name is Zhang Zhidong, according to the US Department of Justice. Arrested in Mexico in 2024, Zhang later made a dramatic escape before he was recaptured and extradited to the US in 2025.

Fentanyl is a synthetic opioid 50 times more potent than heroin. It kills tens of thousands of people each year, mostly in the US, where the finished drug often ends up. A dose as small as a few grains of salt can be lethal.

US President Donald Trump has labelled fentanyl dealers 'narco-terrorists', classified the drug and its components as weapons of mass destruction, and used the fentanyl trade as a reason for imposing tariffs on China, Mexico and Canada.

When Zhang appeared in court in New York in 2025, the Deputy Attorney General at the time, Todd Blanche, described him as one of 'the world's most dangerous traffickers'. He also accused him of 'running a global enterprise that pumped massive quantities of cocaine, fentanyl, and methamphetamine' into the US and laundering 'millions in narcotics proceeds'.

Zhang has pleaded not guilty and is now awaiting trial. We contacted his lawyer, who declined to comment while the case was ongoing.

Cartel members and former colleagues agreed to speak to the BBC to give a rare glimpse into how they believe Zhang - a graduate of China's most prestigious university - allegedly became a key link in the chain between Chinese chemical manufacturers and Mexican drugs laboratories.

Zhang graduated from the prestigious Peking University in Beijing with a Spanish degree in 2010, and a year later travelled to Mexico to work for a Chinese-owned company that mined iron ore. He soon secured a senior role.

Those that knew him at the time saw him as a bright young professional, with an appetite for life abroad. 'He was capable of negotiating with people, very resourceful, and able to adapt to all kinds of environments,' says Alex – not his real name - who studied at the same university and later worked in the same mining company as Zhang in Mexico.

He says Zhang spoke excellent Spanish, with an instinct for street language and the ability to talk to anyone – always with a strong Beijing accent.

Alex says doing business in Mexico sometimes involved dealing with the underworld, including the cartels, which control significant areas of the country. Zhang was able to establish relationships with 'whoever mattered locally - both the official side and the unofficial side', Alex says.

Zhang loved this aspect of Mexico, according to Alex, who paints a picture of a man drawn to risk and recklessness. He recalls him crashing his boss's car, unconcerned about repercussions, and describes how Zhang drove him out of town one night to shoot pistols at road signs on a deserted highway.

In 2013, the mining company collapsed and Alex returned to China but Zhang stayed in Mexico.

Alex says that a year or two later Zhang began to post on the Peking University Spanish alumni group on WeChat, offering to change dollars at preferable rates. Alex believes he was laundering money.

In addition, cartel member Enrique says Zhang also got involved in drugs. Court filings in the US accuse Zhang of operating 'a massive narcotics trafficking and money laundering organization' since June 2016.

Enrique believes Zhang got into a romantic relationship with a female relative of one of the cartel's leaders and suggests this helped him become close to its inner circle.

Another cartel member who ran errands for the organisation, Luis - not his real name - recalls a hot afternoon in 2019, when his bosses asked him to stand guard for a meeting where Zhang 'came to offer his products'. Luis says these products were the precursors – the chemical building blocks – needed to manufacture fentanyl. He sees Zhang as the person who effectively introduced him to fentanyl and started this side of the group's business.

Luis says he soon became a fentanyl cook, making the drug in a clandestine laboratory. He says has seen at least five other cooks die in front of him, and believes this is because the substances they were handling seeped through gaps in their protective clothing. 'Sometimes people just pass out, and we have to carry them out of the room,' he says.

Enrique describes how orders for precursors would be placed with Zhang, who he says used his contacts in China to secure the chemicals.