Norwegian officials are working with foreign intelligence agencies to see if there was any international involvement in the bomb and shooting that killed 92 people.
A Norwegian man has been charged under Norway's counter terror laws over the two attacks on Friday - one near the government's headquarters in Oslo and the other at a youth camp organised by the ruling Labour Party.
At least 85 people were killed at the camp on Utoeya island, 40km from Oslo.
The suspect bought six tonnes of fertiliser from a farm supply firm in May.
The BBC reports speculation has been rife that fertiliser could have been used in the Oslo bomb.
Police say they are investigating whether the attacks were the work of one man or whether others helped.
NRK journalist Ole Torp told the BBC the gunman went to the island dressed in a police uniform, asked people to gather round and then started shooting.
The gunman is reported to have been armed with two weapons, one of them an automatic rifle.
Details are emerging about the accused.
Anders Behring Breivik, 32, is reported to be a Christian fundamentalist who had expressed extreme right-wing views.
He was previously unknown to the police. He is charged with committing acts of terrorism.
The suspect's lawyer said he admitted responsibility for the attacks.
Geir Lippestad told NRK television his client believed his actions were ''atrocious'' but ''necessary'', adding that he would explain himself in court on Monday.
Still pictures of the accused, wearing a wetsuit and carrying an automatic weapon, appeared in a 12-minute video called Knights Templar 2083, which appeared briefly on YouTube.
Police chief Sveinung Sponheim said officers took 45 minutes to reach Utoeya island and the gunman was apprehended 45 minutes after that.
He surrendered immediately and admitted using a weapon,
However, the BBC reports that details of the operation to capture the suspect remain unclear.