Motherboard broke down the FBI's hacking process as follows: The bureau seized the server running Playpen in February 2015, but didn't shut it down immediately. Instead, the FBI took "unprecedented" measures and ran the site via its own servers from February 20th to March 4th, at the same time deploying a hacking tool known internally as a network investigative technique. The NIT identified at least 1,300 IP addresses belonging to visitors of the site.

"Basically, if you visited the homepage and started to sign up for a membership, or started to log in, the warrant authorized deployment of the NIT," a public defender for one of the accused told Motherboard. He said he expected at least 1,500 court cases to stem from this one investigation, and called the operation an "extraordinary expansion of government surveillance and its use of illegal search methods on a massive scale," Motherboard reported.

It's no secret that the FBI uses hacking techniques, including taking advantage of zero-day security holes, and it has an interest in the Dark Web. There's no information on whether the Playpen NIT targeted a zero-day flaw or other known exploit.

[Image credit Matt McClain/The Washington Post/Getty Images]