"He just came out of nowhere," a police official said.
Public broadcaster NRK and several other Norwegian media identified the suspected attacker as Anders Behring Breivik, a blond and blue-eyed Norwegian who expressed right-wing and anti-Muslim views on the Internet.
Norwegian news agency NTB said Breivik legally owned several firearms and belonged to a gun club. He ran an agricultural firm growing vegetables, an enterprise that could have helped him secure large amounts of fertiliser, a potential ingredient in bombs.
But he didn't belong to any known factions in Norway's small and splintered extreme right movement, and had no criminal record except for some minor offences, the police official told AP.
"He hasn't been on our radar, which he would have been if was active in the neo-Nazi groups in Norway," he said. "But he still could be inspired by their ideology." He spoke on condition of anonymity because those details had not been officially released by police. He declined to name the suspect.
Neo-Nazi groups carried out a series of murders and robberies in Scandinavia in the 1990s but have since kept a low profile.
"They have a lack of leadership. We have pretty much control of those groups," the police official said.
Breivik's registered address is at a four-story apartment building in western Oslo. A police car was parked outside the brick building early Saturday, with officers protecting the entrance.
National police chief Sveinung Sponheim told public broadcaster NRK that the gunman's Internet postings "suggest that he has some political traits directed toward the right, and anti-Muslim views, but whether that was a motivation for the actual act remains to be seen."
A Facebook page under Breivik's name was taken down late Friday. A Twitter account under his name had only one Tweet, on July 17, loosely citing English philosopher John Stuart Mill: "One person with a belief is equal to the force of 100,000 who have only interests." Police where interrogating the man, first at the scene of the shooting, and later at a police station in Oslo.
"It's strange that he didn't kill himself, like the guys that have carried out school shootings," the police official told AP.
"It's a good thing that he didn't because then we might get some answers pointing out his motivation." He said the attacks appeared to be the work of a lone madman, without links to any international terrorist networks.
The attack "is probably more Norway's Oklahoma City than it is Norway's World Trade Center," he said referring to the 1995 attack on a federal building in Oklahoma City by domestic terrorists.