I saw that the leader connected to IRC without using TOR and they traced their IP address, but the identity of each member was supposed to be unknown to the others. So even if the leader was acting as an informant, how did the other people get caught?
closed as not constructive by Terry Chia, Adnan, Jeff Ferland♦ May 17 at 6:03
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Despite the fact these guys claimed to be 'anonymous', they were not actually anonymous and happily communicated in open environments (even if they were, for the most part, behind proxies etc). They all had handles associated with their identified. As a result, it was possible to slowly build up a profile and identify the suspects. One they correctly identified sabu, they flipped him and convinced him to coerce the others into providing information that would reveal them.
Some key failings as highlighted here as well as a fairly detailed writeup about it all:
Another presentation worth viewing which provides evidence used in the trials as excerpts here