Doctors will be making an assessment of Anders Behring Breivik's mental health, to see whether he is fit for trial. What might they be considering?
According to the police and Mr Breivik's own lawyer, the suspect has admitted the killings but has not accepted criminal responsibility for them. In other words, he does not believe he should be punished for them.
His lawyer, Geir Lippestad, told Norwegian media on Sunday: "He thought it was gruesome having to commit these acts, but in his head, they were necessary. He wished to attack society and the structure of society."
Mr Breivik is believed to be linked to far-right groups, and it has been suggested that he spent years planning the attacks.
But Professor Jeremy Coid, professor of forensic psychiatry at Queen Mary college, University of London, says that there is likely to be a deep level of mental disturbance underlying this ideology.
"The bottom line is that we don't at this moment know enough about his motives to diagnose his mental state. However, while there are all sorts of cross-cutting with right-wing ideology, I believe he is likely to be suffering from a mental disorder.
"He is across the borderline from extreme ideology. But the devil is in the detail and we need to understand more about his motives."
Norwegian investigators have been probing the far-right views of Mr Breivik - who released a 1,500-page document explaining some of that ideology.
Ian Stephen, a forensic clinical psychologist, told the BBC that it gave an insight into Mr Breivik's character.
"It's one of the scariest documents I've ever read," Dr Stephen said.
"It's written by a man who's absolutely meticulous in his development of his philosophy and he's researched everything, obviously shut away for a long period of time reading, researching, digging into the internet, reading books and formulated this absolute policy of hatred of anything that is Nordic in a sense and looking at planning how to take over the world totally converted in a sort of very rather insane over-complicated deluded manner."
Professor Coid draws parallels with the London "Nail Bomber", whose attacks in 1999 were aimed predominantly at London's black and gay communities.
During David Copeland's trial, the court heard he told police he was a Nazi, and believed in a master race.
The mental state of 23-year-old Copeland had been assessed at Broadmoor Hospital, where he was diagnosed as suffering from paranoid schizophrenia.
However, at his trial at the Old Bailey in 2000 for murder, the prosecution refused to accept his plea of guilty to manslaughter on the grounds that he was suffering a mental illness.
"The Norway attack is on the same lines - where extreme right-wing beliefs merge with paranoid psychosis, or delusional disorder," says Professor Coid.
"In terms of delusional disorder, sometimes the belief that someone starts out with is not particularly bizarre, and these people often get missed. Over time, it becomes apparent that what is going into their head is enormously wrong."
Experts will explore different aspects of Mr Breivik's personality.
"This is someone who clearly has an enormous rage - and the source of that rage will need to be uncovered," says Prof Coid.
He explains that there are basically three diagnoses that could be made: that Mr Breivik is suffering from paranoid psychosis - or a delusional disorder - that he has a severe narcissistic personality or that he is has a schizoid personality disorder.
'Handsome and well-groomed'
"In terms of people suffering from a psychotic illness, everything appears to be focused on the beliefs that they hold," says Prof Coid.
Experts will also try to analyse how Mr Breivik perceives his identity. Some pictures have shown him wearing freemason and military regalia. Others have depicted him as handsome and well-groomed.
"He looks very grandiose and grandiosity is often a feature of a delusional and paranoid personality," says Prof Coid.
But, experts might also look at whether he has a narcissistic personality, says Prof Coid.
"They will want to determine whether he gets off on publicity and being the centre of attention. He obviously enjoys wearing uniforms and medals. But when someone with a narcissistic personality commits an atrocity, it is often after some wound to their psyche, and the attack could be narcissistic revenge - for example against a boss, etc.
"But this case appears to be on a much deeper level - all indications are of a deeper sense of delusional disorder."
Professor Coid says key to the diagnostics would be whether or not Mr Breivik had any close friendships - such as a girlfriend.
"I would be interested in whether in the past he has had normal close friendships, and whether his mental state has deteriorated since then. If he hasn't had any, then maybe we are talking about someone who is closer to the Unabomber, who was cut off from society."
There have been suggestions that Mr Breivik lifted words and ideas from the Unabomber, Ted Kaczynski, who is in prison in the US for sending bombs through the post that killed three people and injured 23 others, from the 1970s to the 1990s.
If we are talking about someone who was cut off socially, "this would indicate a more schizoid personality - an highly functioning individual with a form of autism, and an increasing obsession with right-wing ideology.
"I will put my money of him being a deluded and paranoid individual, however.
"But, in court, if you were to put forward that he was mentally ill, you would have difficulty with prosecution accepting this. People assume mentally ill people are chaotic and lack control.
"Mr, Breivik, however, appeared focused and organised."