Six dozen people were charged with participating in an online child pornography ring that spanned five continents, the U.S. Justice Department said Wednesday. Attorney General Eric Holder and Department of Homeland Security Secretary Janet Napolitano announced the unsealing of three indictments and one complaint charging 72 people for their participation in an international criminal network that promoted pedophilia and encouraged the sexual abuse of very young children, the department said in a release. Operation Delego, an ongoing investigation started in December 2009, targeted the 72 individuals and more than 500 others worldwide for their participation in Dreamboard, a private, members-only, online bulletin board that promoted sexual exploitation of children in a way that avoided law enforcement detection, the Justice Department said. To date, 52 of the 72 charged defendants have been arrested in the United States and abroad. Thirteen have pleaded guilty. The Justice Department said Dreamboard members traded graphic images and videos of adults molesting children no older than 12 years and created a library of images of child sexual abuse. Operation Delego is the largest prosecution to date in the United States of individuals participating in an online bulletin board developed and operated to promote child sexual abuse, distribute child pornography and evade law enforcement, Holder said. \"The members of this criminal network shared a demented dream to create the pre-eminent online community for the promotion of child sexual exploitation but for the children they victimized, this was nothing short of a nightmare,\" Holder said. All 72 of the defendants are charged with conspiring to advertise and distribute child pornography. Fifty also were charged with engaging in a child pornography enterprise. The charges and arrests were conducted in three phases over the course of the operation, Justice Department officials said.