(CNN) -- The man accused of killing 77 people in a terrorist rampage that shook Norway last summer is insane and cannot be sentenced to prison or preventive detention, but can be confined to a mental hospital for the rest of his life, police said Tuesday.
Anders Behring Breivik suffers "grandiose delusions" and "believes he is chosen to decide who is to live and who is to die," Prosecutor Svein Holden announced.
Police said psychiatrists had determined that the 32-year-old man was psychotic at the time of the attacks and during 13 interviews experts conducted with him afterward. The doctors also found him to be paranoid and schizophrenic, police said.
The experts reached their conclusions after 36 hours of interviews with Breivik, police said.
The extension of Breivik's confinement under a compulsory mental health care order will be reviewed by a court every three years, police said. The court will consider whether he still represents a danger to society.
Breivik had not been told of the psychiatrists' findings, police said. His lawyers were expected to relay the news.
The decision underscores the difference between the justice system in the United States and that in Norway, said James Alan Fox, a criminologist at Northeastern University in Boston.
"In the United States, it is extremely difficult to establish successfully an insanity defense," he told CNN in a telephone interview. "You can be paranoid, yet still able to control your behavior and be legally responsible."
In cases like the mass killing in Norway, "there tends to be a tremendous amount of pressure from the public and maybe politicians as well to lock someone away for as long as possible, and bring justice to the victims," Fox said.
Breivik is accused of killing eight people in Oslo and 69 who were among 700 mostly young people attending a Labour Party youth camp on nearby Utoya Island.
He has pleaded not guilty but admits carrying out the attacks, the judge handling his case has said.
Breivik is described by authorities as a right-wing Christian extremist. A 1,500-page manifesto attributed to Breivik posted on the Internet is critical of Muslim immigration and European liberalism, including the Labour Party.
The manifesto predicts that a "European civil war" will lead to the execution of "cultural Marxists" and the banishment of Muslims.
Prime Minister Jens Stoltenberg said in the aftermath of the attacks that his country had been fundamentally changed, but vowed that Norway would remain "an open society."
Oslo Mayor Fabian Stang said Norwegians would respond to the violence with equally powerful weapons: "We're going to punish him with democracy and love," he told CNN shortly after the killings.
Still, memories of the slaughter on the island, where many of the campers survived by hiding behind rocks, remained acute on Tuesday.
"I will always have mixed feelings when I go back, of course," Labour Party member Eskil Pedersen told a reporter. "I think about the 22nd of July and the dreadful things that happened that day, but I have been here every year and every summer since 2000. A lot of my time as a youth has been here on this island. I have very many good memories as well."
He said a memorial will be built there. "Our aim is to reclaim Utoya, take it back as an island, have activities, have a summer camp here," he said.
Relatives of some of the victims expressed disappointment at the decision on Breivik, a journalist from the Norwegian Broadcasting Corp. told CNN. "They have looked forward to seeing him getting a sentence for the rest of his life for the acts he has committed," correspondent Tomm Kristiansen said.
Public reaction is mixed, said journalist Olav Mellingsaeter. Most people are surprised, not angered, by the findings, he said.
"We must trust that the psychiatrists have done a thorough job," a 30-year-old student told the reporter.
A 36-year-old woman said, "As long as he does not escape, I do not care where he's kept."
At his trial in April, Breivik will have the opportunity to present evidence, police said.
He has been in custody since his arrest on Utoya Island on the day of the killings, which marked the deadliest attack in Norway since World War II.
CNN's Tom Watkins, Richard Allen-Greene and Laura Smith-Spark contributed to this report.