An Anonymous hacktivist claiming responsibility for shutting down at least 10,000 dark websites and stealing personal data says he did so because of rampant child pornography on the network.

The attack last Thursday targeted Freedom Hosting II, the largest host of underground websites that can only be accessed through covert web browser Tor. The dark web system where users’ identities are usually hidden is popular with child pornographers.

According to the hacker who claimed responsibility for the operation to Motherboard, more than 50 percent of all files hosted on Freedom Hosting II servers were related to child pornography.

The hacking was particularly notable because it affected about 20 percent of the dark web, and it successfully cracked a system which is generally thought of as being protected by relatively sophisticated barriers.

Users were stunned to find a message from the Anonymous crew announcing, “Hello Freedom Hosting II, you have been hacked,” the Telegraph reports.

“We are disappointed,” the message added. “This is an excerpt from your front page, ‘We have a zero tolerance policy for child pornography.’ All your files have been copied and your database has been dumped.”

The note concluded, “We are Anonymous. We do not forgive. We do not forget. You should have expected us.” The message noted that the operation was “surprisingly easy.”

Besides the message marking pages, hackers demanded .1 Bitcoin (valued at $100) to return stolen data, including the emails of close to 400,000 users. Because the amount called for was relatively small, it wasn’t clear if the demand was serious.

The hacker told Motherboard it was his first operation. He said he just wanted to access Freedom Hosting, but decided to shut it down when he discovered how much child pornography was being distributed.

Anonymous hackers in 2011 infiltrated the original Freedom Hosting, which housed about 40 child porn sites and published the names of users. The cyberattack was part of Operation Dark Lord to crack down on child pornography. The action triggered law enforcement involvement which resulted in several prosecutions. In 2014, Anonymous hackers again targeted child porn in Operation Death Eaters.

The hacking group also threatened President Donald Trump last month, saying he would “regret the next four years.”