Jeremy Hammond was a computer hacker who attacked Stratfor firm. Once FBI was able to pinpoint the real identity of the person behind one of the nicknames who claimed they hacked Startfor, they started extensive investigation on Hammond.
While sup_g may indeed have been a "credible threat," 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. "sup_g" was one of Hammond's nicknames.
Why would FBI go to such great lengths if they already revealed the identity of the perpetrator? Why FBI considered it not enough to correlate an Internet nickname to a physical person? If they required evidence, why did they just not search Hammond's computers after making sure one of aliases that claimed to have hacked Startfor is indeed Jeremy Hammond?