No clue at all, there have been no recent threats. There are always some who are unhappy about our involvement in Afghanistan, of course, but this came out of the blue. However, after the attack in Stockholm I was to a certain extent expecting something like this. If there is a group behind it it's probably some extreme Islamists, I don't know of any political groups that would be interested.
I just hope those who did it are arrested quickly, otherwise there will certainly be some unhealthy paranoia in the population. If Norway hadn't lost its innocence before, it happened today.