Could it be eco-terrorists? Yes. Was there a recent change in policy or approach that this might have been in protest to? That's what I'd look into.
Could it have been a right-wing group? Sure. Have their been other Norwegian right wing terrorist acts? That's what I'd look into.
As for McVeigh, if it happened a year ago it would have likely been initially suspected that Islamists were responsible. However if it happened a year and 4 months ago - say on the anniversary of Waco then perhaps another motivation would have risen to the top. See, there was a direct link between the bombing and government actions.
So, here we have a recent controversial action against an Islamic figure along with fairly recent publishing of cartoons deemed highly offensive to Islam. Couple that with 10+ years of ongoing Islamic terrorist bombings and I think it's reasonable to put Islamic terrorists at the top of the suspect list.
That doesn't mean you conclude they did it; it does mean that without further info they are at the top of the suspect list.