Nearly four million accounts on Adult Friend Finder, a casual dating site that enables users to meet for sex, have been hacked. User's names, email addresses, whether an individual was single and other sensitive details were compromised in the breach, with authorities suggesting the hackers were motivated by the opportunity to blackmail embarrassed users.

An investigation by Britain's Channel 4 News revealed Thursday that 3.9 million of Adult Friend Finder's 64 million users (more than both and eHarmony) had their information stolen and leaked to a hacker forum. It looks like hackers launched a phishing campaign, which includes sending malware infected emails in bulk, and convinced users to enter their log-in information. From there, thieves can use that information to steal a target's identity, or in this case threaten to reveal they're listed as “seeking an extramarital affairs” unless they pay up.

Initial reports say deleted accounts have also been affected.

The site describes itself as a “thriving sexual community” that emphasizes the physical, erotic aspect of romance, rather than helping people find a lasting connection. The site has been criticized for listing far more men than women and for enabling prostitution.

Parent company Friend Finder Networks Inc said in a statement it “understands and fully appreciates the seriousness of the issue. We have already begun working closely with law enforcement and have launched a comprehensive investigation with the help of a leading third-party forensics expert,” as quoted by Business Insider.