John Lewis, FBI deputy assistant director for counter terrorism told a Senate subcommittee that regarding animal rights and environment activists: "There is nothing else going on in this country over the last several years that is racking up the high number of violent crimes and terrorist actions."
He told this to them yesterday (19 May 2005). Not prior to 9/11. Not prior to the Oklahoma City bombing. He said this yesterday.
He acknowledged that they haven't taken human life - yet. It's just been luck so far.
He also mentioned that the FBI drew comparisons with rightwing extremists, KKK, and anti-abortion groups. And apparently found that they stacked up well against animal rights and environmentalists.
(I couldn't find any reference to this in online U.S. media.)