A screen grab of the Twitter account reported to belong to Anders Behring Breivik with a single message that reads, "One person with a belief is equal to the force of 100 000 who have only interests," July 23, 2011.
But at a news conference Saturday, Prime Minister Jens Stoltenberg said it is too soon to draw any conclusions about what motivated the attack.
"Compared to other countries I would not say that we have a big problem with right-wing extremists in Norway," Stoltenberg said. "But we have some groups - we have followed them before - and our police is aware that there are some right-wing extreme groups or at least has been some groups of that kind in Norway."
Footage of youth summer camp, Oslo bombing, and Prime Minister Jens Stoltenberg
Two separate violent attacks took place on Friday. In mid-afternoon at least one bomb exploded near the Prime Minister's offices in central Oslo. Within hours, at least one gunman opened fire at a youth camp on nearby Otoeya island.
Young people aged 14 to 18 were gathered for a summer camp organized by the Prime Minister's Labor Party. They fled in panic as the gunman, dressed as a police man, opened fire, reportedly shooting at random. Video shows bodies floating in the water around the island.
Norway's national news agency NTB said Saturday that witnesses on the island have told police two people were involved. They said the man already in custody was disguised as a policeman, wearing a sweater with a police emblem on it, but that the second man was not.
Media reports in Norway also say the Oslo bomb was made from fertilizer and that a supplier linked a company owned by Breivik to the purchase of fertilizer.
Prime Minister Stoltenberg says the country is united in grief.
"Norway is a small country, but we are a proud country, and we are all very close especially in the times like this," Stoltenberg said. "And I think that all Norwegians feel very close to those who are victims of the violence in Oslo and the youth camp of the young Labor party Otoeya."
On Saturday, Stoltenberg traveled to a hotel near the island, where rescue teams have been transporting survivors. Meanwhile, Emergency teams in downtown Oslo are also still at work looking for bodies and survivors of the massive blast that tore through the Prime Minister's office and the finance ministry.
Mikal Hem, a journalist based in Oslo, told VOA the explosion could be heard from many kilometers away.
"We heard an explosion and the building was shaking," said Hem. "I ran to the window and I saw smoke coming up from the Prime Minister's office."
He says walking past the site later, he saw buildings in tatters.
Hem says Norwegians are in mourning, a grief he says is heightened by the youth of those targeted on the island.
"This is the most terrible incident in Norway since the war so people are shocked and disturbed and sad," Hem added. "These are people as young as 14, 15 years old. It's just terrible."
In an annual report released earlier this year, Norway's police security agency noted a rise in far-right activity, but said that growth was limited by a lack of strong leadership.
This is the deadliest attack in Europe since 2004, when a train bombing in Madrid killed 191 people.