German police have arrested three men and a fourth is being held in Paraguay for allegedly running one of the world's biggest online networks for sharing images of child sex abuse.
The international operation, involving several police forces, targeted a dark net platform called Boystown, which has now been taken down.
Officials say Boystown had more than 400,000 registered users.
They say some images showed the most serious sexual abuse of young children.
The dark net is an internet area beyond the reach of mainstream search engines.
The German-led investigation involved law enforcement agencies in the Netherlands, Sweden, Australia, the US and Canada.
The EU police agency Europol says it will also examine intelligence gained from the operation, and "more arrests and rescues are to be expected globally" on that basis.
Several related paedophile chat sites on the dark net were also dismantled, Europol says.
German police say three men detained since mid-April are believed to have run the paedophile network: one aged 40 from Paderborn, one aged 49 from the Munich area and a 58-year-old from north Germany who has been living in South America for several years. He was detained in Paraguay's Concepción region under an international arrest warrant, and German authorities have asked for his extradition.
The fourth suspect is a 64-year-old from Hamburg who was allegedly one of the most active members, responsible for more than 3,500 posts.
Police catching up fast
The dark web is a notorious layer of the web that many people never see.
You won't find its pages on mainstream search engines such as Google - you'll need a specialised web browser like Tor to access it at all.
Once you're in, if you know how to look, you'll find all sorts of activity.
The dark web itself is not illegal and neither is being on it. Indeed it is used by some for whistleblowing and activism in parts of the world where censorship is rife and such behaviour is heavily punished.
But there is plenty of material on it that is absolutely outside of the law.
There are forums where harrowing images of child sexual abuse and other extreme explicit content is shared or traded. In the many illegal marketplaces you'll also find banned drugs and weapons for sale.
Anonymous sellers spring up as quickly as they disappear. Payment is generally in cryptocurrency and there are no consumer rights or customer support here if the merchant - or indeed the entire store - suddenly vanishes.
It happens, and it's not unusual. That's in part because law enforcement is getting better targeting at these operations. I've met several experts who specialise in tracking illegal traders across the dark web - and, much like all criminals, cyber criminals also make mistakes.
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