The US has charged 72 people it alleges were part of a global child pornography network used to share photos and videos of sexual abuse.
Announcing charges after 20 months of Operation Delego, senior US officials said some 600 users of a site known as "Dreamboard" had been investigated.
Attorney General Eric Holder said some images on the site showed the abuse of infants and young children.
The US has 43 of those charged in custody, with nine held overseas.
The 20 others charged as part of the US probe remain at large, Homeland Security Secretary Janet Napolitano said, and are known only by their internet pseudonyms.
BBC News - US charges 72 over global child porn operation