U.S. and European authorities said Thursday that 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 in early July at the same time the authorities arrested the reported founder of the site, Alexandre Cazes, a Canadian man who was living in Bangkok.

Cazes killed himself in his jail cell shortly after he was arrested, authorities said Thursday. He was 25 years old.

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 in June 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 in June and accused of operating Hansa Market.

AlphaBay and Hansa Market were successors to the first and most famous market operating on the so-called darknet, Silk Road, which 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 came under scrutiny because many of its vendors sell synthetic opioids, like fentanyl, which play a central role in the nationwide overdose epidemic.

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 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 Thursday. “Most of this activity was for illegal drugs, pouring fuel on the fire of the national illegal drug epidemic.”

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.