Earlier this month, Alexandre Cazes, a computer whiz from Quebec, was living in Bangkok. He had a villa in Phuket, a Lamborghini and a Porsche, along with bank accounts in Thailand, Liechtenstein and Switzerland.

But the 25-year-old millionaire planted the seed of his downfall years ago, when he left his Hotmail e-mail address visible online. It was that clue which led police to detain him as the alleged mastermind of AlphaBay, a Dark Web site that served as the world's biggest anonymous online marketplace of illegal goods.

Within days of his arrest on July 5, Mr. Cazes was found dead hanging in a Bangkok jail. His apparent suicide occurred ahead of his extradition to the United States, where he was indicted on 16 criminal counts, including racketeering, narcotics conspiracy and money laundering.

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Details of the investigation that led to Mr. Cazes's demise and the dismantling of AlphaBay are outlined in U.S. court documents released Thursday. This case amounts to the second time this year that American federal agents have successfully targeted an ostentatiously wealthy Canadian computer expert in his early 20s while unfurling a global conspiracy.

U.S. Attorney-General Jeff Sessions announced the charges in Washington, telling reporters the AlphaBay bust stands as one of the most significant criminal investigations of 2017. He said such marketplaces are directly linked to a deadly continuing opioid epidemic, which is now killing one American every 11 minutes.

"We know of several Americans who were killed by drugs [bought] on AlphaBay," Mr. Sessions said. "One was just 18 years old when in February she overdosed on a powerful synthetic opioid which she had bought on AlphaBay."

The RCMP assisted the U.S. Federal Bureau of Investigation in its probe of Mr. Cazes and started a parallel criminal investigation of him after being tipped by U.S. authorities in January.

While the Mounties have lately been redoubling their own efforts at tackling cybercriminals and halting fentanyl smuggling, a chief superintendent has publicly lamented how Canadian efforts have been stalled by clandestine transactions done anonymously on the Internet, through sites that bear a close resemblance to AlphaBay.

In March, an alleged hacker-for-hire from Ancaster, Ont., was arrested on a U.S. indictment alleging that he hacked customer information from computer giant Yahoo on behalf of Russian intelligence agents. Until that point, 22-year-old Karim Baratov was simply known as a wealthy Ontario resident who had somehow amassed a Lamborghini, a Porsche 911 and an Aston Martin.

According to the U.S. indictment against Mr. Cazes, he had a $23-million (U.S.) net worth. A U.S. asset-seizure order issued against him and his Thai wife includes a Lamborghini Aventador, a Porsche Panamera, a Mini Cooper, a BMW motorcycle, and accounts with four Thai banks, Bank Alpinum AG in Liechtenstein and Bitcoin Suisse AG, in Baar, Switzerland.

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The filings also said the couple owned real estate in Bangkok, in the resort town of Phuket, in Cyprus and in the Caribbean state of Antigua and Barbuda. Mr. Cazes obtained citizenship in Antigua, the documents say, thanks to his $400,000 purchase of a beachfront property. They also say he was in the process of acquiring Cypriot citizenship by spending €2.4-million ($3.5-million) to buy a villa in Famagusta, a picturesque port on the east coast of Cyprus.

"The payments for these illegal items," the forfeiture complaint said, "represent racketeering proceeds based on their connection to the AlphaBay marketplace."

Mr. Cazes was born Oct. 19, 1991, and he grew up just outside Trois-Rivières, 150 kilometres downriver from Montreal. His father, Martin, a garage owner, described his son as a trouble-free child who was so gifted that he skipped a year ahead in school. "An extraordinary young man, no problems, no criminal record," the elder Mr. Cazes recently told the TVA news network. "He never smoked a cigarette, never used drugs."

Alexandre Cazes was 17 when he founded his own business, EBX Technologies, which he incorporated as a company selling software and repairing computers. The family believed that he had made his fortune by transacting in digital currencies, but according to the U.S. indictment, EBX Technologies was simply a front.

At the time the FBI shut it down, AlphaBay allegedly carried 250,000 listings for illegal drugs and chemicals and more than 100,000 listings for stolen items. This dwarves the 14,000 listings that had existed on Silk Road, another infamous Dark Web marketplace that was dismantled by the FBI in 2013. In fact, Mr. Cazes launched AlphaBay just a few months after Silk Road was put out of business, according to the U.S. indictment in Eastern California District Court.

Like its predecessor, AlphaBay operated on the Dark Web, which means the marketplace could be accessed only through Tor, an increasingly popular software that enables anonymized Internet communications that are very difficult to trace, even for government-intelligence agents. In addition to selling drugs, AlphaBay users also sold weapons, computer malware and stolen credit-card information.

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According to the court complaint, the site charged a commission between 2 per cent and 4 per cent.

AlphaBay grew to have about 10 employees, the court documents say. "We take no responsibility if you get caught," the site allegedly warned, "so protecting yourself is your responsibility."

U.S. authorities began investigating AlphaBay by tasking federal agents to make undercover purchases of illicit drugs, fake ID documents and a device that steals card data from bank-machine users.

The turning point in the probe came late last year. That's when investigators discovered that during an early phase of AlphaBay's operations, it displayed an administrator's e-mail address when users initiated a password-recovery process.

That e-mail – "Pimp_Alex_91@hotmail.com" – belonged to Mr. Cazes, according to U.S. federal agents. They discovered that when he was 17, Mr. Cazes used the same e-mail account to post a virus-removal tip on a tech forum.

Last month, U.S. authorities obtained warrants against Mr. Cazes, who was at his home in Bangkok when it was raided by the Royal Thai Police. They found an open laptop computer in his bedroom that was logged on AlphaBay as an administrator, the complaint said. Inside the laptop were passwords, a list of AlphaBay servers and a file detailing assets.

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A week after his arrest, and just before his extradition hearing, Mr. Cazes was found dead. According to the Bangkok Post, he used a towel to hang himself from the toilet door of his cell.

In Quebec, Martin Cazes can't comprehend that Alexandre's life ended in such a fashion. "In my heart as a father, it's hard to accept that my son committed suicide," he told TVA. "He was under police supervision. It's unbelievable."