German prosecutors are investigating 12,000 suspected members of a child sex abuse network on the internet - the biggest in the country's history.
An internet provider had suspicions about a large exchange of data
A senior public prosecutor said the suspects were accused of downloading or possessing illegal images of children.
The investigation, which has been going on for several months, also points to suspects in about 70 other countries.
An internet provider in Berlin is said to have helped by alerting the inquiry to a huge amount of internet traffic.
"The material was analysed. Then we called for search warrants," said Peter Vogt, head of the central office tackling child internet sex abuse. He was speaking to German radio station Mitteldeutscher Rundfunk (MDR).
The suspects include 300 under investigation in the eastern German state of Sachsen-Anhalt.