Bertrand Monnet is a professor at EDHEC, a business school, where he is the director of the Criminal Risks Management Chair. In this capacity, he has been interested in the Sinaloa cartel since 2014. It took him years to identify intermediaries capable of guaranteeing both the reliability of his interlocutors and his own safety. This investigation into fentanyl, which resulted in a three-part video series broadcast on Le Monde, required numerous trips to Mexico and a long process of building trust. "The narcos agreed to speak out of a desire to display their power on the international stage," said Monnet.

"Dubai is a paradise for us!" In the heart of Business Bay, the emirate's business district, Eduardo, a Mexican "narco," interrupts his selfie session in front of the Burj Khalifa tower, the world's tallest skyscraper, to explain to me how the various clans of the Sinaloa cartel come to launder tens of millions of dollars here. He works for one of them in a dual capacity: He exports tons of drugs, whether cocaine or fentanyl, an opioid 30 times more potent than heroin, and he then launders the profits from this idiosyncratic line of business.

Several months of investigation in Culiacan, the cartel's stronghold in Mexico, and then in New York, one of its main markets, gave me a precise idea of the value chain in the trafficking of fentanyl, the hot product of the moment and the primary cause of the 130,000 overdose deaths in the US in 2022. Bought for $17,000 (€15,700) in China, a kilo of fentanyl powder gets transformed in Culiacan's secret laboratories into blue pellets called M30. The clans that make up the organization – several dozen in all – enjoy a form of autonomy in producing this drug and then selling it, at $400,000 a kilo, to American traffickers, who in turn sell it in millions of doses of a few milligrams, sometimes mixed with other narcotics, such as heroin, cocaine. For a clan like Eduardo's, which is particularly active in the business of fentanyl, trafficking generates enormous revenues. For the cartel and its 19,000 members, the profits amount to billions of dollars, which are, of course, laundered.

A few weeks earlier, in Culiacan, this same Eduardo deciphered, in a way, his organization's money-laundering methods to me. It wasn't anything too complicated or too secretive. The first phase of the operation, placement, consists of distributing the cash – in concrete terms, heaps of banknotes – into thousands of bank accounts. To do this, the cartels either inject the money into the treasuries of the hundreds of companies and businesses they control or deposit it directly into bank branches whose employees they bribe so that they won't report these deposits as suspicious.

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