The Oslo attacks come as European counterterrorism officials say terror groups are shifting their targets to countries where attacks have been less common, and perhaps more unexpected.

A confidential June report by the European police agency Europol described the shift. Favored targets include Scandinavian countries, one senior intelligence official said, noting a string of recent incidents.

In December, a suspected suicide bomber blew himself up among Christmas shoppers near a busy street in the center of Stockholm.

In September, an Iraqi Kurd, one of three men arrested in July in the Oslo area and in the German city of Duisburg, confessed to planning a terrorist strike that may have targeted Jyllands-Posten, the Danish newspaper known for publishing controversial cartoons of the Prophet Muhammad....