re: More US leftist-terrorist groups emerge
The ELF has long been this kind of threat, and while many more moderate environmentalists disavow them, there is still a lot more tolerance for their activities in many areas than there would be for a less politically-correct activist group.
Some people on the left claim that they aren't really "terrorists", since they do primarily property damage. Of course the left has little conception of the connection between life and property, but this excuse is still bullshit.
The front page picture from the ELF Website.
Also please note the other proud arson links, and the arson lesson book.