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Oslo, Norway (CNN) -- The suspect in the deadliest attack in Norway since World War II has acknowledged carrying out the mass shooting and bombing and claims to have worked with two other cells, a judge said Monday.

Judge Kim Heger said the suspect, Anders Behring Breivik, acknowledges carrying out Friday's attacks, but has said they were necessary to prevent the "colonization" of the country by Muslims. Breivik accused the Labour Party, whose members were targets of the mass shooting, of "treason" for promoting multiculturalism, the judge said.

Police refused to release information about their investigation into the possibility that two cells aided Breivik, saying Monday that a court hearing was closed so as not to disclose any evidentiary information.

Other court officials have said they could not confirm the existence of the cells and referred questions to police.

During his court hearing Monday, Breivik appeared "very calm," a police official said. "He was very concise in trying to explain why he was trying to do this," the official said. "But when he started reading from his manifest, he was stopped."

Two court psychiatrists will be assigned to the case, he added.

Monday's hearing was closed to the public for "security reasons and because of a concern that it would impede the investigation," court communications director Irene Ramm told CNN.

Afterward, Heger told reporters that he had ordered Breivik to remain in custody for eight weeks, until his next scheduled court appearance. Authorities continue to investigate the bombing in Oslo and the mass shooting at a nearby island that together killed at least 76 people. If police need more time, they can petition the court for it, he said.

Authorities initially said 93 had died but announced Monday that eight people were confirmed dead in the bombing and 68 in the shooting. "Some people might have been counted two times," a police official told reporters about the lowered toll.

Police were still searching Monday in and around Utoya Island for shooting victims, with 50 officers -- some of them using cadaver dogs -- combing through the crime scene for any remaining casualties.

The suspect will be held in isolation for the next four weeks to ensure he has no opportunity to tamper with evidence, Heger said. Breivik has access to his lawyer but to no one else, and not to letters or news, court officials said.

Breivik asked to wear a uniform to the hearing but was not allowed to, Heger said.

Breivik, 32, is a suspected right-wing Christian extremist who appears to have written a 1,500-page manifesto ranting against Muslims and laying out meticulous plans to prepare for the attacks.

CNN has not independently confirmed that Breivik is the author of the manifesto, which bears his name and says it is intended to be circulated among sympathizers.

The Norwegian government called for a national moment of silence Monday, ordering trains halted as part of a nationwide observance to remember the victims of Friday's bombing in downtown Oslo and shooting at a political youth retreat on Utoya Island.

Court officials were among many who stood in silence to mark the moment at noon.

Police spokesman Henning Holtaas told CNN that the suspect was charged with two acts of terrorism, one for the bombing and one for the mass shooting.

In Norway, the maximum sentence for such a charge is 21 years. However, the court could assess an extension if the person was still deemed a threat after having served the sentence, he said.

Breivik, a Norwegian, had told investigators that he acted alone and was not aided in the planning, acting National Police Chief Sveinung Sponheim told reporters Sunday.

Sponheim said investigators were studying a manifesto published online the day of the attack.

The suspect told investigators during interviews that he belonged to an international order, The Knights Templar, according to the Norwegian newspaper VG, which cited unidentified sources.

He described the organization as an armed Christian order, fighting to rid the West of Islamic suppression, the newspaper said. He also told investigators he had been in contact with like-minded individuals and said he counts himself as a representative of this order, it said.

Holtaas declined to confirm the news report, saying, "We are not commenting on such details."

The newspaper report mirrors statements in the manifesto.

The manifesto contains photographs of Breivik wearing what appears to be a military uniform that features an altered U.S. Marine Corps dress jacket with Knights Templar medals.

The Knights Templar were Christian Crusaders who helped fight against Muslim rule of the Holy Land in the Middle Ages, but the order was shut down 700 years ago.

The manifesto with Breivik's name on it refers to a "European Military Order and Criminal Tribunal (the PCCTS -- Knights Templar) ... created by and for the free indigenous peoples of Europe" in London in 2002.

It rants against Muslims and their growing presence in Europe and calls for a European civil war to overthrow governments, end multiculturalism and execute "cultural Marxists."

Authorities allege that Breivik killed eight people Friday by setting off a car bomb in downtown Oslo that targeted government buildings, then traveled 20 miles to Utoya Island and killed 68 teens and young adults who were attending a political youth retreat.

The suspect was carrying a considerable amount of ammunition when he surrendered to authorities on the island, Sponheim told reporters.

Investigators will conduct autopsies over the next few days, Sponheim said, and the identities of the victims will be released once all the next of kin have been notified.

The youth camp is an annual tradition in the Labour Party, Prime Minister Jens Stoltenberg said in an interview. "I myself have participated every summer since 1974, since I was a teenager," he said.

He predicted its impact will be long-lasting, but will not fundamentally change the country. "We will have a Norway before and a Norway after the bomb and the killings," he said. Still, he added, "I will do whatever I can to make sure that ... Norway will be possible to recognize; that even after these terrible incidents, (it) will be an open society, will be a democratic society."

Among those killed on the island was Trond Berntsen, the step-brother of Crown Princess Mette-Marit, according to a statement released by the Royal House Communications office.

At least four people have not been accounted for around Utoya Island, with investigators searching the waters nearby for victims who may have drowned trying to escape the shooter.

CNN's Jonathan Wald, Nic Robertson, Michael Holmes, Jennifer Deaton, Erin McLaughlin, Chelsea J. Carter and Greg Botelho contributed to this report.