Police said Anders Behring Breivik was psychotic at the time of the attacks and during 13 interviews with experts, CNN reported.
Police said the experts reached their conclusion based on 36 hours of interviews with Breivik.
Breivik is accused of killing eight people in a bombing in downtown Oslo and 69 people attending a political retreat during a shooting rampage on nearby Utoya Island. He confessed to the killings.
The 243-page psychiatric report said Breivik, 32, should not stand trial because he was mentally ill and not responsible for his actions, Britain's Sky News reported. The report said he developed paranoid schizophrenia and should be held indefinitely at a psychiatric hospital.
During a news conference, Public Prosecutor Svein Holden said the experts "described a person who lives in his own delusional universe."
The report concluded Breivik "believes he is chosen to decide who is to live and who is to die," Holden said.
The full report will be presented during an April 16 court hearing.