Espen Andersen Brathen, a 38-year-old Dane living in Norway, allegedly used a bow and arrow inside and outside a supermarket before stabbing to death five other residents in the southeastern town of Kongsberg in October.
Brathen entered the pleas when asked to respond to the charges of murder and attempted murder at the court in the town of Hokksund. “Everything, I admit everything,” he said, at the start of his trial.
While Norwegian police had initially suspected some kind of terror attack, they quickly believed it was the work of an unbalanced individual.
Brathen had been living for years in Kongsberg, home to about 25,000 people some 80 kilometres (50 miles) west of the capital Oslo, and authorities have said he has a medical history, although details have not been made public.
The Norwegian security services PST, which are responsible for counter-terrorism, also said the man had been on their radar.
He was arrested 35 minutes after the first reports of an attack and was swiftly moved to a medical institution.
Three experts who observed him concluded that the suspect was suffering from paranoid schizophrenia.
Both the prosecution and the defence argued that he could not be held criminally responsible and advocated a psychiatric commitment rather than a prison sentence.
According to the prosecution, Brathen was armed with a bow, 60 arrows and four knives on the day of the attacks. His victims were four women and one man aged from 52 to 78.
The trial is scheduled to last until June 17, with a verdict expected in the following weeks.