Thai cops arrested Alexandre Cazes in Bangkok on July 5 and had planned to extradite him to the US, where he faced drug trafficking and money laundering charges.
But the computer programmer hanged himself with a towel in his detention cell a week later on July 12, according to Thai anti-narcotics police, who have been tight-lipped on the details of his case.
On Saturday a Thai officer confirmed Cazes was accused of being an "operator" of a major online black market.
"It's a huge dark web market that trafficks drugs and sells other illegal stuff," the police officer said, requesting anonymity.
Speculation is rife that the underground marketplace was AlphaBay, considered the world's largest and most lucrative darknet bazaar until it was taken down within hours of Cazes' arrest.
Like its predecessor Silk Road, which was shut down by the FBI in 2013, AlphaBay used the encrypted Tor network and virtual currencies like Bitcoin to shield customers from detection.
According to Nicolas Christin, a computer science professor at Carnegie Mellon University, AlphaBay gained prominence in 2015 and mostly traded in drugs, stolen credit cards and forged IDs.
It was "more than twice as big as Silk Road was in its heyday, with a revenue of somewhere between 600,000 and 800,000 dollars a day in early 2017, and that's a rather conservative estimate," he told AFP.
Cazes appeared to be living a life of luxury in Thailand, where he owned three houses and four cars -- including a Lamborghini -- according to Thai police who have seized the assets.
He slipped into the country around eight years ago, according to police.
"He was a computer expert involved with international transactions of Bitcoins," said Major General Soontorn Chalermkiat, a spokesman for Thailand's Narcotics Suppression Bureau.
"He didn't have any business in Thailand but he had many houses," the officer said, adding that Cazes' Thai wife has since been charged with money laundering.
The US Embassy in Bangkok refused to comment on the details of the case, saying only that Cazes was detained at the request of the US "with a view toward extradition to face federal criminal charges".
The founder of Silk Road -- the world's first and most famous digital drug market -- was sentenced to life in prison by federal court in New York in 2015.
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