The breach was first reported by U.K. news outlet Channel 4, which said information from nearly 4 million AdultFriendFinder users was lost in the incident. Hackers reportedly stole data about users’ sexual preferences, email addresses, usernames, dates of birth and zip codes.

AdultFriendFinder told USA Today that it has “begun working closely with law enforcement and launched a comprehensive investigation with the help of leading third-party forensics expert, Mandiant.” Channel 4 reports that some of the data was linked to users who had attempted to delete their profiles on the site.

AdultFriendFinder can best be described as an adult-themed social network meant to connect individuals for short-term encounters.