Federal law enforcement officials say 72 people have been charged with participating in an online international network engaged in the sexual abuse of children.
Attorney General Eric Holder and Homeland Security Secretary Janet Napolitano said Wednesday a 20-month law enforcement effort called Operation Delego targeted more than 600 people around the world since the probe was launched in December 2009.
Those charged were involved in Dreamboard - a private, members-only, bulletin board created to promote pedophilia.
According to documents released in the probe, the members allegedly traded tens of thousands of graphic images and videos of children 12 years old and under.
At a news conference at the Justice Department, the attorney general called the criminal activity a "nightmare for the children."
"Some of the children featured in the images and videos were just infants," Holder said.
Napolitano said the amount of child porn swapped by participants in the network was massive, equivalent to 16,000 DVDs.
Authorities have arrested people in 13 other countries - Canada, Denmark, Ecuador, France, Germany, Hungary, Kenya, the Netherlands, the Philippines, Qatar, Serbia, Sweden and Switzerland. But some of those were arrested on local rather than the U.S. charges.