Police attorney Christian Hatlo said investigators are increasingly confident that the right-wing extremist had no accomplices when he set off a bomb in Oslo's government district and opened fire at a political youth camp outside the capital.
As a result, they won't ask for his solitary confinement to be extended when it expires next week, Hatlo told reporters in Oslo.
"We consider it safe to take him off isolation," Hatlo said.
However, he added that the decision means little in practice for the 32-year-old Norwegian, because authorities at the Ila prison still want to keep him isolated from other prisoners for security reasons. A ban on newspapers, visitors, correspondence, Internet and TV access is in effect until Nov. 14.
Hatlo said investigators found 186 ammunition shells on Utoya island, the ruling Labor Party retreat where police say Breivik killed 69 people attending a summer camp for the party's youth wing.
In a 1,500-page manifesto posted online before the attacks, Breivik called for a revolution to purge Europe of Muslims and punish politicians who have embraced immigration.
But Hatlo said police have found no evidence to support Breivik's claims that he belongs to a network of modern-day crusaders opposed to multiculturalism, and that two other cells are ready to strike.
"We are becoming more and more certain that no two cells exist in the way that he describes," Hatlo said.
He said police have questioned Breivik on 14 occasions and believe that he had the knowledge and skills to put together the explosives used in the Oslo bombing on his own.
Still, police have asked authorities in about 20 countries to bring in people for questioning to find out more about Breivik's contacts and his purchases of bomb-making chemicals, Hatlo said.
Norway's top prosecutor Tor-Aksel Busch has said he aims to start the trial around Easter next year.