"The strongest hypothesis after the first days of the investigation is that illness is in the background," police inspector Per Thomas Omholt told reporters two days after the attack.
On Friday a Norwegian court also ordered the man who confessed to killing five people in an attack to be held in detention in a medical facility, as questions mounted about his mental health.
Espen Andersen Brathen, a Danish citizen who converted to Islam and is believed to have been radicalised, will be held for an initial period of four weeks, the first two in total isolation, judge Ann Mikalsen ruled.
A full psychiatric evaluation -- which can take several months, according to the prosecutor -- is necessary to determine whether Brathen can be held legally responsible for his actions.
"This indicates that things are not exactly as they should be," his lawyer, Fredrik Neumann said referring to his client's mental health.
"A complete judicial assessment will clarify that," he told Norwegian daily VG.
Brathen was not present in court on Friday, having not contested the detention request. While authorities have said the attack appeared to be an act of terror, they have not ruled out that it may have been the act of a mentally unstable person.
"There is no doubt that (it) appears as if it could be an act of terror, but it's important that the investigation continues and that we establish the motive of the suspect," the head of Norway's intelligence service PST, Hans Sverre Sjovold, told a news conference on Thursday.
"This is a person who has been in and out of the health system for some time," Sjovold said.
Known to police previously over fears he had radicalised, Brathen has confessed to the killings in questioning, police said.
Four women and one man were killed and three people injured, and police said a bow and arrows and other undisclosed weapons were used before he was arrested.
"He has told us why he did it but we can't say anything publicly about his motives at this stage," prosecutor Ann Iren Svane Mathiassen told AFP on Thursday.
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