Someone else should step up and give a comprehensive overview, because I can only recount the timeline from memory, not provide sources. But iirc, Anonymous was a loose coalition of random people on the internet, some of which turned out to have some basic hacking skills. They weren't really taken seriously until they embarrassed HBGary. At that point, the FBI began investigating them. Anonymous changed their name to LulzSec (or possibly AntiSec). The FBI used standard police techniques to infiltrate and eventually dismantle the those groups. The technique was simply to 1) figure out who was a member of Anonymous, 2) threaten them with prosecution unless they cooperate, 3) repeat. This eventually led them to turn Sabu, Anonymous's leader. I'm going to dig for an interesting HN comment I remember reading about the relationship between FBI, Anonymous, and the takedown of Freedom Hosting.

EDIT: Here it is: http://news.ycombinator.com/item?id=6154642

In order to be friendly to mobile users, I'll copy-paste the comment here (although the link is worth reading because of the informative replies):

---

redthrowaway 40 days ago | link | parent | flag

Interesting. Freedom Hosting had been a target of Anonymous' Operation Darknet from the beginning--they're well-known for refusing to take down exploitative sites. Operation Darknet is, itself, a pretty interesting phenomenon: Anonymous hacks onion sites, then hands over user information to the FBI for investigation. Anonymous does what the FBI legally can't, and in exchange they're not prosecuted for it. I can't find the article now, but I recall reading an interview with an FBI agent in Wired or Ars or some such where he described the anons as "Internet Superheroes". (sic)

That, in and of itself, is kind of curious. Curiouser? One of the original Op Darknet principals was Sabu. You may remember him as the hacker the FBI rolled and got to bust up LulzSec. Sabu was turned by the FBI on June 7th, 2011.[1] Operation Darknet began several months later, in October, 2011.[2]

The obvious question, then, is this: Did the FBI use Sabu to entice Anons into attacking child porn networks, thereby evading the laws against them doing it themselves? Did they use the fact they turned a well-known hacktivist to help them deal with criminals they lacked the legal tools to go after? Is this arrest the culmination of those efforts?

[1] https://en.wikipedia.org/wiki/Sabu_(hacktivist)

[2] http://www.informationweek.com/security/attacks/anonymous-at....