If a court agrees with that assessment, the self-declared anti-Muslim militant cannot be sentenced to prison but will be subjected to compulsory psychiatric care, prosecutors told reporters in Oslo.
"The conclusions of the forensic experts is that Anders Behring Breivik was insane," prosecutor Svein Holden said, adding Breivik was in a state of psychosis during Norway's worst peacetime massacre.
In Norway, an insanity defense requires that a defendant be in a state of psychosis while committing the crime with which he or she is charged. That means the defendant has lost contact with reality to the point that he's no longer in control of his own actions.
The 243-page report will be reviewed by a panel from the Norwegian Board of Forensic Medicine, which could ask for additional information and add its own opinions.
The head of that panel told AP in July that it was unlikely that Breivik would be declared legally insane because the attacks were so carefully planned and executed.