US officials Wednesday unveiled charges against 72 people in their largest global probe into Internet child pornography which smashed a \"nightmare\" online bulletin board catering to pedophiles.The investigation launched in 2009 has led to arrests in the US and 13 other countries of participants in Dreamboard, which had a \"VIP\" ranking system for members trading in graphic images and videos of adults molesting children age 12 and under, often violently, the Justice Department said.\"Dreamboard\'s creators and members lived all over the world -- but they allegedly were united by a disturbing belief that the sexual abuse of children is proper conduct that should not be criminalized,\" Attorney General Eric Holder said.\"The members of this criminal network shared a demented dream to create the preeminent online community for the promotion of child sexual exploitation, but for the children they victimized, this was nothing short of a nightmare.\"The ongoing probe has led to the arrest of 52 people in the US and 13 other countries -- Canada, Denmark, Ecuador, France, Germany, Hungary, Kenya, the Netherlands, the Philippines, Qatar, Serbia, Sweden and Switzerland.Thirteen of the 52 individuals arrested have pleaded guilty, and 20 of the 72 individuals charged \"remain at large and are known only by their online identities,\" a Justice Department statement said.Operation Delego \"represents the largest prosecution to date in the United States of individuals who participated in an online bulletin board conceived and operated for the sole purpose of promoting child sexual abuse, disseminating child pornography and evading law enforcement,\" it said.\"Dreamboard was a self-described global \'community\' of pedophiles dedicated to the relentless victimization and exploitation of children 12 and under,\" said Assistant Attorney General Lanny Breuer.According to the Justice Department, membership was tightly controlled by the site\'s administrators, who required prospective members to upload child pornography when applying for membership. Members were then required continually to upload images of child sexual abuse in order to keep up their membership in the board, which included rules of conduct printed in English, Russian, Japanese and Spanish.Some of the children featured in the images and videos were just babies, said Holder. \"And, in many cases, the children being victimized were in obvious, and intentional, pain -- even \'in distress and crying,\' just as the rules for one area of the bulletin board mandated,\" he added.All 72 of the defendants are charged with conspiring to advertise and distribute child pornography, and 50 are also charged with engaging in a child pornography enterprise. \"Dreamboard members allegedly used the power and anonymity of the Internet to motivate each other to commit their horrific acts of sexual abuse of minors and trading in child pornography,\" Breuer said.According to court documents, members employed a variety of methods to conceal their activities, using \"screen names\" and proxy servers, which can be used to reroute web traffic and disguise a user\'s actual location.Dreamboard members also encouraged the use of encryption programs on their computers, which password-protect computer files to prevent law enforcement from accessing them in the event of a court-authorized search.Officials said the charges and arrests were conducted in three separate phases over the course of the operation.Four of the 13 individuals who have pleaded guilty for their roles in the conspiracy have been sentenced to prison.On May 10, Timothy Lee Gentry, 33, of Burlington, Kentucky, was sentenced to 25 years in prison; on May 31, Michael Biggs, 32, of Orlando, Florida, was sentenced to 20 years; on June 22, Michael Childs, 49, of Huntsville, Alabama, was sentenced to 30 years; and on July 14, Charles Christian, 49, of Tilton, Illinois, was sentenced to more than 22 years in prison. Operation Delego was conducted by the Justice Department with Homeland Security\'s Immigration and Customs Enforcement, along with other global law enforcement agencies including Eurojust, the European Union\'s Judicial Cooperation Unit and \"dozens of law enforcement agencies throughout the world,\" the statement said.