Top executives at what was once one of the world's most popular file-sharing sites called themselves "modern pirates" and acknowledged in Skype conversations they made money from users sharing unauthorized copies of movies and videos, according to documents released Friday by federal prosecutors.

In 191 pages of evidence, the U.S. Justice Department laid out its case against Megaupload Ltd., the company behind a "cyberlocker" site that allowed users to upload files and is considered by the entertainment...