The man suspected of killing five people with a bow and arrows and other weapons in Norway has been transferred to the public health service, a state prosecutor has said, amid concerns about his mental health.

“Based on an assessment of his health condition, this was the best solution,” the prosecutor, Ann Iren Svane Mathiassen, told the Norwegian public broadcaster, NRK, on Friday, shortly before a local court agreed to the prosecution’s request for four weeks of pre-trial detention.

Police have said the man, identified as Danish citizen Espen Andersen Bråthen, 37, was a convert to Islam who had petty crime and drug convictions and was once flagged for suspected radicalisation, but serious doubts have been raised about whether he could be held legally responsible for the attack.

A psychiatric evaluation, which could last several months, began on Thursday. The Norwegian security service PST has said the attack “appears to be an act of terror”, but emphasised that only a full investigation could establish Bråthen’s motive.

The head of the agency, Hans Sverre Sjøvold, said the suspect, who police have said has confessed to the killings and is cooperating with the investigation, “has been in and out of the health system for some time … We have to spend some time on that history here, and it’s important the investigation gets it straight.”

NRK reported that Bråthen had several previous convictions for robbery and drugs offences, and was last year handed a six-month restraining order banning him from approaching two close family members after he threatened to kill one of them.

An unnamed relative dismissed the reports of his radicalisation as irrelevant, describing Bråthen to the broadcaster as “mentally ill” and adding that that the family had been receiving threats for several years. “This is about a person who is seriously mentally ill and who became marginalised from adolescence. This has seriously affected the lives of those who are close to him,” the relative said.

Four women and one man aged between 50 and 70 were killed in the attack on Wednesday evening in the town of Kongsberg, 41 miles (66km) south-west of Oslo. Three people, including an off-duty police officer, were injured, and discharged from hospital on Friday.

The attack started in a supermarket and continued over “a large area” of the town, police said. It lasted about 35 minutes before Bråthen, armed with a competition bow and arrows and other weapons reportedly including a knife, was arrested.

Mathiassen said there was no reason to believe the attack was planned in advance, nor anything to indicate that “a particular situation in the supermarket was responsible for triggering them”.

Norway’s new prime minister, Jonas Gahr Støre, who took office on Thursday after winning elections last month, is due to visit the town on Friday with the justice minister, Emilie Enger Mehl.

The death toll was the worst of any attack in Norway since 2011, when the far-right extremist Anders Behring Breivik killed 77 people, most of them teenagers, at a youth camp on the island of Utøya.

A memorial service for the victims will be held in Kongsberg church on Sunday.