The American and European authorities said on 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 dark net 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.

Mr. Cazes committed suicide in his jail cell shortly after he was arrested, the authorities said on 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 the vendors and customers and gathering identifying details on those involved in the 50,000 transactions that took place.