German police arrested three men on Monday on suspicion of operating one of the world’s largest child pornography internet platforms following a series of raids across the country.

The men are accused of operating the platform as administrators on the so-called “darknet”, which is not visible to normal users and can only be accessed using specific software and authorisations, the Federal Criminal Police Office (BKA) said.

Investigators from both the BKA and the Frankfurt public prosecutor’s office searched seven properties in three states, North Rhine-Westphalia, Bavaria and Hamburg, before making the arrests, dpa news agency quoted officials as saying.

Another suspected member of the ring was arrested in Paraguay at the request of German law enforcement authorities, the BKA said.

The three main suspects are a 40-year-old man from the west German city of Paderborn, a 49-year-old man from the Munich area and a 58-year-old man from northern Germany who has been living in South America for several years, investigators said.

The man arrested in Paraguay, a 64-year-old from Hamburg, is suspected of having posted more than 3,500 articles as one of the most active users of the platform.