The latest word we heard here in Oz was that it was a local act committed by a local far right extremist member or possibly group or as you Norwegians call them "Neo-Quislings".
Some kind of neo nazi collaborant ?
If the media claims about his political views and motives are true and him having a connection to a Swedish based Neo-Nazi group the term neo-quisling probably suits him well. But again i don't trust what media say first off until the Police confirm the truth from their investigation findings unless the media have successfully hacked into the guy that the police arrested computers,email/website accounts and tapped his phones to obtain the information.
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"The British Air Farce have droopped their bums on the water works..They have scored a direct hot on the pimps"- British agent/gendarme Officer Crabtree, "Allo,Allo."