They had to piece together Jeremy Hammond
Jeremy Hammond was arrested and charged with being one of the computer hackers who stole credit card numbers from the security company Stratfor.
The FBI did two things: first, they caught and "turned" another member of Anonymous, Hector Xavier Monsegur (known as "Sabu"), who became an undercover informant. This meant that all of Mr. Hammond’s online activities with Sabu were monitored and recorded.
Second, they pieced together small scraps of information to link his online name ("sup_g" ) to his other aliases and his real-world activities:
…he was in the end no match for the overwhelming federal resources of the FBI agents hunting him down. Over the last month, federal agents staked out his home in Chicago constantly, dug up old police surveillance records, tapped his Internet connection, used directional wireless finders to locate and identify his wireless router, and relied on Sabu back in his New York City apartment to let them know when sup_g went on or offline.
Read more: Stakeout: how the FBI tracked and busted a Chicago Anon, Ars Technica>>
– Chicago man accused in international hacking bust, Chicago Sun Times>>