“DO NOT MAKE NEW ORDERS ON ANY DNM ANY MORE!” a popular post on the social network Reddit said on Thursday, using initials for dark net markets.
“You got this one law enforcement,” the post said.
AlphaBay and Hansa Market were successors to the first and most famous market operating on the so-called dark net, 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 Justice Department said Thursday.
The site recently come 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. The sale of such drugs on AlphaBay was detailed in a front-page article in The New York Times last month.
Hansa Market had about 1,800 vendors selling drugs of all sorts, the Dutch authorities said on Thursday.
“This is likely one of the most important criminal investigations of this entire year, I have no doubt about that,” Attorney General Jeff Sessions said at a news conference on Thursday. “Most of this activity was for illegal drugs, pouring fuel on the fire of the national illegal drug epidemic.”
AlphaBay went down in early July, prompting speculation that it had been seized by law enforcement authorities.
The man said to be the founder and leading operator of the site, Mr. Cazes, was arrested at his home in Bangkok on July 5, officials said on Thursday.
Mr. Cazes, originally from Canada, had his laptop open and was logged in to AlphaBay at the time of his arrest, allowing the authorities to gain access to all of AlphaBay’s hidden servers and financial accounts, according to legal documents unsealed on Thursday.
Access to AlphaBay ceased at the same time as Mr. Cazes’s arrest, but the authorities did not immediately take credit for the operation, and some AlphaBay administrators said they were working to get the site back up.
Dark net 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.
Officials said they found Mr. Cazes because he had posted his personal email address, Pimp_Alex_91@hotmail.com, in some early messages from AlphaBay.
Mr. Cazes began working on AlphaBay in the summer of 2013, before Silk Road was taken down, and brought it online in December 2013, an indictment unsealed on Thursday said.
Mr. Cazes had help from about 10 deputies, who administered the site and resolved disputes, the documents said. The 10 were not identified, but authorities said the investigation was continuing.
The AlphaBay takedown involved officials from six countries and Europol, led by the American authorities. The Dutch police led the takedown of Hansa Market, but both efforts were coordinated under the code name Operation Bayonet. It was not the first time that the United States and European authorities worked together on online black markets.
The 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.
“In the past weeks, the police have intercepted tens of thousands of nonencrypted messages between sellers and buyers about orders,” the Dutch police said in a statement on Thursday. “They were also able to identify the delivery address for a large number of orders. Some 10,000 foreign addresses of Hansa Market buyers were passed on to Europol.”
Hansa Market was open earlier this week and announced that it was planning to ban fentanyl from the site. A Dutch official told the journalist Brian Krebs that the site’s moderators had proposed the ban, and the police, operating undercover, did not oppose the decision.
On Thursday, visitors to AlphaBay and Hansa Market found messages from the authorities announcing the seizure of the sites.
The authorities have moved quickly to seize Mr. Cazes’ significant assets, including properties in Antigua, Cyprus and Thailand; 10 vehicles, including a Lamborghini and a Porsche; and financial assets of about $ 18 million.
Some of Mr. Cazes’ money was in the virtual currencies used on AlphaBay: Bitcoin, Ether and Monero. He also had bank accounts in Liechtenstein, Thailand and Cyprus.
AlphaBay and Hansa Market are the latest in a long line of dark net markets to rise quickly and then get taken down by law enforcement. So far, new contenders have been quick to take over the business.
Even before AlphaBay went down, it had several large competitors. In the last few weeks, a site known as Dream Market has emerged as the leading player.
On Thursday, Dream Market had 57,000 listings for drugs and 4,000 listings for opioids.
“Critics will say that as we shutter one site, another will emerge, and they may be right,” Andrew G. McCabe, the acting director of the Federal Bureau of Investigation, said on Thursday.
“We know that removing top criminals from the infrastructure is not a long-term fix,” he added. “There is always a new player waiting in the wings read to fill those shoes.”
For now, though, users congregating on Reddit were on guard.
“It would be advised to go quiet for a little while,” a user going by the screen name 1ntr4sp3ctive wrote.