The United States has unsealed indictments against 72 people allegedly involved in an international child pornography network that encouraged sexual abuse of children.

U.S. Attorney General Eric Holder and Homeland Security Secretary Janet Napolitano announced the indictments Wednesday. Holder said the announcement is a strong message that people who attempt to exploit children will be found, stopped, and brought to justice.

Holder said the investigation targeted hundreds of individuals around the world. He said 52 people who were indicted have been arrested in 14 different countries. The probe began in December 2009 and is ongoing.

He said the network known as “Dreamboard” required members to upload a steady supply of child pornography and added that people who produced their own child pornography gained elevated status. He said participants were encouraged to display images of children in distress. He said some of the children featured were just infants.

Holder called such actions “unconscionable.”

He said 13 people have already pleaded guilty and four have been sentenced to 20 to 30 years in prison.