Two of the world's most notorious "darknet" marketplaces have been knocked out in a one-two punch that officials say yielded a trove of new intelligence about drugs and weapons merchants that operate from hidden corners of the internet.

AlphaBay, previously the internet's largest darknet site, went offline on July 5 and was already widely assumed to have been taken out by authorities.

But today, European law enforcement revealed that a second darknet site known as Hansa had also been in police hands for the past month, an announcement deliberately designed to sow panic among tech-savvy dealers and buyers.

"This is the largest darknet marketplace takedown in history," Attorney General Jeff Sessions told reporters in Washington, according to a prepared version of his statement.

He accused online vendors of "pouring fuel on the fire of the national drug epidemic" and warned that "the darknet is not a place to hide."

The darknet, a part of the internet accessible only through specialised anonymity-providing tools, is an attractive place for sales of drugs, weapons and other illegal merchandise.

Merchants and buyers can operate with relative openness while still keeping their identities secret.

A California indictment named AlphaBay's founder as Canadian Alexandre Cazes, saying the 25-year-old had amassed a fortune of US$23m, 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.

Cazes died in police custody just over a week ago, according to Thai police.