It’ll take awhile for Norwegian law enforcement to sort it all out and identify a particular group. As usual, many will take credit.
They have captured a gunman alive on the island (according to the Norwegian paper)...maybe they can extract useful info from him.
It just has all the earmarks of Al Quaeda (or whatever name the Jihadists use in Europe nowadays). Multiple attacks, ongoing, coordinated, soft targets.
What could be a softer target than 500 kids on school vacation, enjoying a summer camp on an island?
Whoever did it—they are despicable cowards.