Oslo - Anders Behring Breivik told a Norwegian judge on Monday his bombing and shooting rampage that killed scores aimed to save Europe from a Muslim takeover, and said that "two more cells" existed in his organisation.
Breivik's remarks at the closed-door custody hearing were relayed by the judge, Kim Heger, at a news conference.
The killer has previously said he acted alone and police had earlier said they were trying to confirm this.
But after Breivik's claim about other cells, police attorney Christian Hatlo said "we cannot completely rule out" the possibility that others were involved in Friday's attacks.
Police revised the death toll downwards to 76 from 93, saying eight people were now known to have died in the bomb blast in central Oslo, and 68 on the island of Utoeya.
It was not clear whether Breivik is in fact part of an organisation, although he has written about a revival of the Knights Templar, a medieval order of crusading monks.
After the hearing, Heger said he had ordered Breivik detained in solitary confinement for eight weeks, with no letters, newspapers or visits, except from a lawyer.
The detention, in line with a request from prosecutors, will allow them to investigate the case against Breivik.
Jeering crowds awaited Breivik at Oslo District Court.
"Everyone here wants him dead," Roeine said, adding that he knew one of the dead and three survivors of the attacks.
Breivik had wanted to explain in public why he perpetrated modern-day Norway's worst peace-time massacre. He was denied a public platform, but the judge, in his news conference, gave an account of what the accused 32-year-old had said.
Breivik says attack meant to deter Labor Party recruitment.
Heger said Breivik had accused the ruling Labour Party of betraying Norway with "mass imports of Muslims".
He said his bombing of government buildings in Oslo and massacre at a summer camp for Labour's youth wing was aimed at deterring future recruitment to the party.
"The goal of the attack was to give a strong signal to the people," the judge quoted Breivik as saying.
Breivik's custody can be extended before his trial on terrorism charges. Police say the trial could be a year away.
After the hearing, a police jeep drove away carrying an unshaven Breivik, with close-cropped blond hair and wearing a red jumper with a lighter red shirt underneath.
Breivik planted a bomb on Friday outside Stoltenberg's Oslo office which killed eight, then drove to the wooded island of Utoeya and shot dead 68 at the Labour Party youth camp.
In a rambling 1,500-page tract posted online just before the massacre, Breivik explained how violence was needed to rescue Europe from Islam, immigration and multi-culturalism.
If he survived his assault and was arrested, this would "mark the initiation of the propaganda phase", he wrote.
The judge's decision to close the hearing to the public followed an outcry from Norwegians enraged at the possibility that Breivik would be allowed a public platform for his views.
The maximum jail term in Norway is 21 years, although that can be extended if there is a risk of repeat offences. "In theory he can be in jail for the rest of his life," said Staale Eskeland, professor of criminal law at the University of Oslo.
