The case opened shortly after Steven Chase launched Playpen in the summer of 2014. The FBI, which has numerous investigations involving the dark web, quickly became aware of the site, but “given the nature of how Tor hidden services work, there was not much we could do about it,” Alfin recalled.

That is, until December 2014, when Chase slipped up and revealed Playpen’s unique IP address—a location in the U.S. The gaffe was noticed by a foreign law enforcement agency, which notified the FBI.

“From that point we took normal investigative steps—seized a copy of the website, served search warrants for e-mail accounts, followed the money—and everything led back to Steven Chase,” said Alfin. Chase was sentenced Monday in North Carolina in connection with engaging in a child exploitation enterprise and multiple child pornography charges. His sentencing follows those of two co-defendants who were also administrators on the website—Michael Fluckiger, 46, of Indiana, and David Browning, 47, of Kentucky—who were each given 20-year prison terms earlier this year.

Arresting Playpen’s administrators, however, was only the beginning. In January 2015, the FBI, in partnership with the Department of Justice Child Exploitation and Obscenity Section, launched Operation Pacifier—an effort to go after Playpen’s thousands of members. Using a court-approved network investigative technique, agents uncovered IP addresses and other information that helped locate and identify users. Investigators sent more than 1,000 leads to FBI field offices around the country and thousands more to overseas partners, Alfin said.

Arrests and other enforcement actions have occurred in countries far and near. Europol, the European Union's agency for law enforcement cooperation​, reported arrests, along with Israel, Turkey, Peru, Malaysia, Chile, and the Ukraine. International agencies critical to the investigation included CNCPO Polizia Postale e Comunicazioni of Italian State Police​, the United Kingdom’s National Crime Agency, and New Zealand's Department of Internal Affairs.

Even some countries where law enforcement cooperation has been historically limited were, in this case, especially helpful in pursuing the FBI’s leads on former users and contributors to Chase’s Playpen site.

“Members of his enterprise who were raping children, who were producing child pornography all around the world—those cases continue to be indicted and prosecuted,” Alfin said.

In addition to taking down the website, the ongoing investigation, as of May 4, 2017, has produced the following results:

At least 350 U.S.-based individuals arrested

25 producers of child pornography prosecuted

51 hands-on abusers prosecuted

55 American children successfully identified or rescued

548 international arrests, with 296 sexually abused children identified or rescued

The Playpen site has been down for more than two years. But similar sites continue to operate and proliferate on the dark web.

“It’s ongoing and we continue to address the threat to the best of our abilities,” said Alfin. “It’s the same with any criminal violation: As they get smarter, we adapt, we find them. It’s a cat-and-mouse game, except it’s not a game. Kids are being abused, and it’s our job to stop that.”