WASHINGTON • US and European authorities said they had shut down two of the largest online black markets, AlphaBay and Hansa Market, and arrested their operators.
AlphaBay, the largest so-called darknet market, was taken down earlier this month at the same time the authorities arrested the reported founder of the site, Alexandre Cazes, a Canadian man who was living in Bangkok.
Cazes was logged on to AlphaBay at the time of his arrest, allowing the authorities to find his passwords and other information about the site's servers, according to legal documents.
Cazes killed himself in his jail cell shortly after he was arrested, the authorities said on Thursday.
He was 25.
After AlphaBay went down, users streamed to one of its largest competitors, Hansa Market.
But on Thursday, the Dutch national police announced that they had taken control of Hansa Market last month and had been operating the site since then, monitoring vendors and customers, and gathering identifying details on those involved in the 50,000 transactions that took place.
Two men were arrested in Germany last month and accused of operating Hansa Market.
The international exercise to seize AlphaBay's servers also involved the authorities in Thailand, Lithuania, Canada, Britain and France.
"This is likely one of the most important criminal investigations of the year - taking down the largest darknet marketplace in history," US Attorney-General Jeff Sessions said at a news briefing in Washington to announce the action.
"Most of this activity was for illegal drugs, pouring fuel on the fire of the national illegal drug epidemic," he said.
AlphaBay and Hansa Market were two of the top three criminal marketplaces on the dark web, Europol chief Rob Wainwright said at the press conference.
The move struck a blow to an international drug trade that has increasingly moved online in recent years, though some experts thought its impact would be limited. "The takedown of AlphaBay is significant, but it's a bit of a whac-a-mole," said Mr Frank Cilluffo, director of the Centre for Cyber and Homeland Security at The George Washington University.
Criminals, he said, "are going to flock to other places".
AlphaBay and Hansa Market were successors to the first and most famous market operating on the so-called darknet, Silk Road, which the authorities took down in October 2013.
AlphaBay grew into a business with 200,000 users and 40,000 vendors - or 10 times the size of Silk Road - the US Justice Department said on Thursday.
The site recently came under scrutiny because many of its vendors sell synthetic opioids, like fentanyl, which play a central role in the nationwide overdose epidemic.
The authorities said 122 vendors had been advertising fentanyl on the site.
Hansa Market had about 1,800 vendors selling drugs of all sorts, Dutch authorities said on Thursday.
Darknet sites are reached with special browsers that obscure the location and identity of the user and the server, making it hard for officials to locate and shut them down.
Dutch authorities said they had been able to use Hansa Market as a trap to catch vendors and customers fleeing AlphaBay.
The authorities said that in the days after AlphaBay went down, the number of vendors operating on Hansa Market jumped to 8,000 on an average day from 1,000.