In early 2018, the F.B.I. created the Joint Criminal Opioid Darknet Enforcement team, or J-Code, with more than a dozen special agents and staff. Europol has its own dedicated dark web team. And authorities everywhere have broadened their focus beyond just the administrators overseeing the markets.

During the first few months of 2019, American officials conducted an operation called SaboTor, which focused on the vendors selling drugs on the dark net. There were 61 arrests in just a few weeks. One ring, in the Los Angeles area, was said to be responsible for shipping 1,500 packages of crack, heroin and methamphetamine each month.

Richard Downing , who oversees the computer crime section of the Justice Department, said he and his colleagues have focused on techniques that create distrust on the sites by encouraging users to believe that sellers and site administrators have already been compromised and are feeding information to law enforcement.

There are some signs that these tactics have limited the growth of the markets. When the authorities took down the Wall Street Market in early May it had 5,400 vendors, only one-seventh as many as AlphaBay had when it was shuttered two years earlier.

Several markets have also chosen to ban the sale of fentanyl, to make them less attractive targets for law enforcement. Berlusconi, one of the largest markets, announced that it was making this change in early May soon after Wall Street Market was shut down.

But the impact of the law enforcement activity of the last two years could be temporary.

Data from Chainalysis suggests that before the latest crackdown, overall transactions on the dark net had recovered to nearly 70 percent of the previous peak, right before AlphaBay went down, and were growing each month.

Mr. Downing, the Justice Department lawyer, said that for now there was a recognition, even among the authorities, that dark net markets have become an enduring part of the criminal economy.