Police had previously said the suspect, Espen Andersen Brathen, was armed with a bow and arrows and two other weapons
Norwegian police said Monday that the five victims of last week's attack were killed by a "sharp object" used by the suspect, not a bow and arrows.
"At some point he discarded or lost his bow and arrows," police inspector Per Thomas Omholt told reporters.
He said that during the attack on Wednesday the suspect killed "five people with a sharp object both in private addresses and in public spaces".
Police, who had previously said that the suspect Espen Andersen Brathen was armed with a bow and arrows and two other weapons, did not specify the nature of the sharp weapons, adding that they were still interviewing witnesses.
"Everything points to the victims being selected at random," Omholt said.
According to the police, more than 10 people were also shot at with arrows at the start of the attack, but none were killed with this weapon.
During police questioning, Brathen has confessed to the killings and to wounding three others.
The 37-year-old Danish citizen has announced publically that he is a convert to Islam and intially police reported that there had been fears of radicalisation.
He is however being kept in a medical facility pending a psychiatric evaluation, which is necessary to determine whether Brathen can be held legally responsible for his actions.
"As far as motive is concerned, illness remains the main hypothesis. And as far as conversion to Islam is concerned, this hypothesis is weakened," Omholt added.
On Saturday, police announced the identities of the five victims, four women and one man: Andrea Meyer, 52, Hanne Merethe Englund, 56, Liv Berit Borge, 75, Gunnar Erling Sauve, 75 and Gun Marith Madsen, 78.