The information about the attack is contained in documents exfiltrated by NSA whistleblower Edward Snowden and shared with journalists, in this case a PowerPoint presentation from 2012 aimed at sharing the success with the NSA.
According to the document, the attack was performed by a GCHQ unit called Joint Threat Research Intelligence Group (JTRIG), and was directed towards an IRC server that hosted chat rooms where Anonymous members congregated and exchanged information. They managed to make it impossible to access for some 30 hours.
Prior to this, members of JTRIG (posing as other Anonymous members, of course) have engaged in conversations with some of the participants, such as the infamous LulzSec spokesman Topiary, and other hackers, several of which were ultimately also arrested.
They have managed to use the information gleaned from those conversations and from booby-trapped websites to which they lured the targets to for discovering the real-life identities of several Anonymous members that engaged in data stealing.
In addition to all this, JTRIG mounted a campaign of messages sent via Facebook, Twitter, email, IM, and Skype to dissuade Anonymous members from participating in DDoS attacks - and were apparently very successful.
The question these revelations raised is whether the GCHQ had the right to perform this DoS attack. The agency says they operated within the limits of the law and with rigorous oversight, but privacy advocates pointed out that not all Anonymous members are criminals, and that the chat rooms were also used to organise protests and campaigns that are not illegal.
"Targeting Anonymous and hacktivists amounts to targeting citizens for expressing their political beliefs,” says Gabriella Coleman, an anthropology professor at McGill University. “Some have rallied around the name to engage in digital civil disobedience, but nothing remotely resembling terrorism. The majority of those embrace the idea primarily for ordinary political expression.”