Police questioned a blond 32-year-old suspect on Saturday over twin attacks on a youth camp and the government headquarters that killed at least 91 people in Norway's deadliest tragedy since World War II.
As harrowing testimony emerged from the holiday island where scores of youngsters were mown down by a gunman dressed as a policeman, Norway's premier said the country would emerge stronger from the "cruel act of violence".
"Never since the Second World War has our country been hit by a crime on this scale," Jens Stoltenberg told journalists in an early morning press conference as police searched for more bodies on the idyllic Utoeya island.
Police commissioner Sveinung Sponheim confirmed that the suspect was a 32-year-old Norwegian who had posted anti-Muslim rhetoric online.
"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 media reported that he described himself on his Facebook page as "conservative", "Christian", and interested in hunting and computer games like World of Warcraft and Modern Warfare 2.
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 said.
"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.
The attacks on Friday afternoon were western Europe's deadliest carnage since the 2004 Madrid bombings.
While there had been initial fears they might have been an act of revenge over Norway's participation in the campaigns in Afghanistan and Libya, the focus shifted when it emerged the suspect was a native Norwegian.
Speaking alongside the prime minister, Justice Minister Knut Storberget said there was no reason to raise the threat level.
Security was tightened across potential target sites in the capital, but police lifted an advisory that had urged residents to stay home.
Seven of the victims were killed in a massive explosion which ripped through government buildings, including Stoltenberg's office and the finance ministry, in downtown Oslo.
But it is thought that the bomber then caught a ferry to nearby Utoeya wearing a police uniform.
According to witness testimony, he claimed to be investigating the bomb attack and began opening fire with an automatic weapon after beckoning youngsters towards him.
Witnesses described scenes of panic and horror among the 560 people attending the youth camp. Some who tried to swin to safety were even shot in the water, according to witnesses.
Among the wounded was Adrian Pracon, who was shot in the left shoulder as the gunman opened fire.
Speaking to Australia's ABC network from hospital, he said the scene on the island was like a "Nazi movie".
"He was shooting people at close range and starting to shoot at us. He stood first 10 metres from me and shooting at people in the water," he said.
"He had an M16, it did look like a machine gun. When I saw him from the side yelling that he was about to kill us, he looked like he was taken from a Nazi movie or something.
"He started shooting at these people, so I laid down and acted as if I was dead. He stood maybe two metres away from me. I could hear him breathing. I could feel the heat of the machine gun.
"He tried everyone, he kicked them to see if they were alive, or he just shot them.
Another young survivor, Jorgen Benone, said: "People were hiding behind stones.
I saw people being shot... I felt it was best to stay quiet, not to run into the open.
"I saw (the gunman) once just 20 to 30 metres away from me," Benone said, adding that he then swam to safety and was rescued by a boat.
Norwegian police said they feared there could also be explosives on the island and officers were combing the island. Their updated death toll on the island released on Saturday morning stood at 84.
Stoltenberg had been due to give a speech on Saturday to the 560 people attending the youth camp on the island, organised by the ruling Labour party.
"Utoeya is a place I have visited every summer since 1974. I have known joy, commitment and safety there. Now the place has been through brutal violence and a paradise for youth has been turned into hell in a few hours," he said.
The prime minister said Norway, one of Europe's most peaceful countries, would not be intimidated.
"People have lived through a nightmare that very few of us can imagine," he said. "The coming days will show who is responsible and what kind of punishment they will get.
"The message to whoever attacked us, the message from all of Norway is that you will not destroy us, you will not destroy our democracy and our ideals for a better world."
There was widespread international condemnation with US President Barack Obama saying the attacks were "a reminder that the entire international community has a stake in preventing this kind of terror from occurring."
The Norwegian capital is a well-known symbol of international peace efforts and home to the Nobel Peace Prize.