A 25-year-old man from Trois-Rivières, Que., who was found dead in a Thai jail last week, was described Thursday as the mastermind behind an illicit dark web empire by the top law-enforcement official in the U.S.

At a news conference in Washington, U.S. Attorney General Jeff Sessions said the effort to take down Alexandre Cazes was "likely one of the most important criminal investigations of this entire year."

Cazes was arrested July 5 in Thailand by local and U.S. authorities. The U.S. Department of Justice alleges he was the creator and administrator of AlphaBay, a website with 40,000 vendors and more than 200,000 customers.

"By far, most of this illegal activity was in drugs," Sessions said.

The dark web is a version of the internet accessed through a special browser where sites aren't indexed, which means they can't be found by search engines. Drugs, guns, stolen credit card information and other illicit goods are often traded on the dark web.

On July 12, Cazes was found dead in his jail cell. In a news release, U.S. Justice officials said he took his own life.

Thai police said Cazes hanged himself in his cell prior to a scheduled court hearing.

Father doubts allegations

According to his father, Martin Cazes, his son had been living in Thailand for four years with his wife.

He doubts his son was the mastermind that he's portrayed as being.

"I'd like your opinion on how the biggest site (240,000 users) can be created, managed and operated by one person, what do you think?" he told CBC News over Facebook, saying he would not comment extensively until an autopsy is performed.

"And, by the way, every time I saw my son, he was enjoying life, he never looked stressed."

He said when his son visited Trois-Rivières, he said his fortune was linked to bitcoin, real estate and playing the stock market.

According to Sessions, Cazes's arrest was a co-ordinated effort between law enforcement authorities in Thailand, the Netherlands, Lithuania, Canada, the United Kingdom and France, as well as the European law enforcement agency Europol.

U.S. Attorney General Jeff Sessions announced the takedown of the dark web marketplace AlphaBay at the Justice Department in Washington. (Aaron P. Bernstein/Reuters)

He said criminals are acting like "they can commit crimes with impunity by going dark."

Sessions said he hopes Cazes's arrest will send a clear message that there is nowhere for criminals to hide.

A California indictment said Cazes had amassed a fortune of $23 million, including a small fortune in digital currency.

The indictment said he spent the money on real estate, luxury cars and the pursuit of "economic citizenship" in Liechtenstein, Thailand and Cyprus, reported The Associated Press.