I'm not linked in to US anarchist circles, and this doesn't seem to have been taken up by mainstream media, so your guess about the offence(s) is as good as mine, although I have tried following this story beyond the above links. As far as I can tell the FBI seem to be pursuing "anarchists" for vandalism at an anti-capitalist protest. They seem to be casting quite a wide net for a story that the media aren't apparently interested in, unless I'm googling wrong or there rly is some sort of super secret conspiracy going on.

Disclaimer: my own experience is that most "anarchists" couldn't organise their way out of a paper bag, quite apart from threatening the state, but there have been a few notable exceptions, obviously. ::wryface::