This wasn't the FBI, it was some other hacker who was pissed at him. The screenshots [1] show that quite clearly. That guy likely just turned xyz over to the feds after doxing him (and rooting him, and taking over most of his online accounts)
What's really funny is that going after an official FBI site would have gotten less of a backlash from real hackers. Attacking Infragard is like slapping a bunch of hackers (white-hat and black) in the face, so they're going to be a lot more motivated to expose the perpetrators. It's all pretty silly.
[1]