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 cells, a judge said Monday.
Judge Kim Heger said the suspect, Anders Behring Breivik, acknowledged carrying out Friday's bombing and shooting, 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.
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. He has pleaded not guilty, police said Monday.
Two court psychiatrists will be assigned to the case, police said.
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 that Breivik 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 people 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.
But officials predicted the official toll could rise again. Police were still searching Monday in and around Utoya Island for victims, with 50 officers -- some of them using cadaver dogs -- combing through the crime scene for any remaining casualties.
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.
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, 32, asked to wear a uniform to the hearing but was not allowed to, Heger said.
The suspected right-wing Christian extremist appears to have written a 1,500-page manifesto that rants against Muslims and lays 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.
As Norwegians tried to come to grips with what had befallen their normally peaceful country, their government called Monday for a national moment of silence, 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.
Nearly 200,000 people participated in a memorial Monday in downtown Oslo to honor the victims, police said.
Oslo Mayor Fabian Stang praised Monday's memorial service as a display by Norwegians that they do not accept violence. "Today, people turned out to show that this town is ours and we don't accept this," he told CNN about the actions attributed to Breivik. "We're going to punish him with democracy and love."
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, which does not have the death penalty, the maximum sentence for such a charge is 21 years. However, the court could impose an extension if the person were deemed still to be a threat after having served the sentence, he said.
Breivik's lawyer, Geir Lippestad, told Norwegian broadcaster NRK that his client is prepared to spend the rest of his life in jail.
Breivik had expected to be tortured by police and shot during Monday's court proceeding, the lawyer said.
Breivik, a Norwegian, had told investigators that he acted alone and was not aided in the planning of the attacks, acting National Police Chief Sveinung Sponheim told reporters Sunday.
Sponheim said investigators were studying the manifesto published online on 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 people 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 bearing Breivik's name 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 rails against Muslims and their growing presence in Europe and calls for a European civil war to overthrow governments, end multiculturalism and execute "cultural Marxists."
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 the attacks' impact may 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."
Espen Barth Eide, the deputy foreign minister, predicted the shock of Friday's events may linger in Norway, but the country will not fundamentally change. "Almost everyone in Norway is now determined to not allow this to change the way we live, and change the way we are -- we are an open and tolerant society."
More security may not be the answer, he said. "There is always a balance between security and risk. If you have more police, you may protect something, but you may lose something. So, there's something valuable in the sort of openness that we will try to protect together."
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.
CNN's Jonathan Wald, Nic Robertson, Michael Holmes, Jennifer Deaton, Erin McLaughlin, Chelsea J. Carter and Greg Botelho contributed to this report.