Four of 13 men who pleaded guilty for their roles in a child pornography ring were sentenced to prison with terms ranging from 20 to 30 years.

Timothy Lee Gentry, 33, of Burlington, Ky., was sentenced to 25 years in prison on May 10. Michael Biggs, 32, of Orlando, Fla., was sentenced to 20 years in prison on May 31, 2011. While Michael Childs, 49, of Huntsville, Ala., was sentenced to 30 years in prison in June and Charles Christian, 49, of Tilton, Ill., will spend more than 22 years in prison, according a press release from the U.S. Department of Justice.

Each defendant also received a lifetime of supervised release as part of their sentence.

The Justice Department announced on Wednesday that 72 people were charged for participating in an international criminal network formed to sexually exploit children. Twenty of them still remain at large, according to the press release.

Those who are still at large are known only by their online identities and efforts are being made to identify them, the department said.

Fifty-two of 72 defendants charged were arrested in the U.S. and abroad and were charged with conspiring to advertise and distribute child pornography. Fifty of them were charged with engaging in a child pornography enterprise.

Authorities launched the ongoing investigation Operation Delego December 2009, targeting the 72 charged defendants and more than 500 others worldwide who took part in Dreamboard, a private, members-only, online bulletin board promoting pedophilia and encourage the sexual abuse of young children.

The justice department said the members traded graphic images and videos of adults molesting children 12 years and under, often violently, and they collectively created a huge private library of images of child sexual abuse.

"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," said Attorney General Eric Holder. "This operation marks another important step forward in our work to protect children across - and beyond - this country. Our nation's fight to protect the rights, interests, and safety of children goes on, and it will continue to be a top priority of this Justice Department."

The department said the operation is the largest prosecution to date in the U.S. of people who participated in an online bulletin board conceived and operated for the sole purpose of promoting sexual abuse of children and publicizing child pornography while evading law enforcement.