Claiming responsibility for an atrocity - truly or falsely - makes tactical sense for an organisation in some circumstances, but not always. It is a way of asserting power - but it comes at a cost.

Circumstances alter cases. Especially where there is widespread abhorrence of what has been done, it can make better sense to avoid any claim of responsibility, but to encourage the idea that the culprit is a lone individual, driven to take extreme action by social and political faults in society - with the implication that similar things can be expected to happen again.

It appears to have been common practice in the Northern Ireland for responsibility for particular atrocities to be laid off to some non-existent grouplet, or just left floating without any claim.

The bottom line here is, we just don't know. The failure of organisations who share Breivik's views to claim responsibility tells us nothing whatsoever. As would be the case equally if there were such claims.