Assessing whether Norway's confessed mass killer Anders Behring Breivik is insane, as his lawyer asserts, will take months of observation, interviews and analysis -- and experts say it's hard to fake mental illness.

Forensic psychiatrists who analyze the minds of murderers point to clear patterns of behavior that can date far back and may give vital clues to Breivik's mental state when he killed at least 76 people in a shooting spree and bomb attack on July 22.

They say the most likely final outcome is that Breivik, 32, will be jailed or kept in a secure mental hospital until he is at least in his early fifties, and any release beyond that would be allowed only if a parole board thinks it is safe -- unlikely given the scale and severity of his crimes.

"The insanity defense is about whether someone understands what they're doing, and if they do..whether they realize it is legally wrong," said Seena Fazel, a clinical senior lecturer in forensic psychiatry at the University of Oxford.

"If you think when you're attacking somebody that you're actually attacking a piece of bread...and if you had no idea what you did was legally wrong...then people would consider that as part of an insanity defense," he told a briefing in London.

These elements also play into another key consideration for insanity -- whether the killer had the capacity to form intent.

"A high degree of planning and premeditation, and an ability to see through to the consequences of your actions in a rational way, imply some capacity to form intent," Fazel said

Breivik's copious writings -- some of which appear in diaries and a 1,500-page "manifesto" in which he wrote of himself as a righteous crusader on a mission to save European "Christendom" from a tide of Islam -- suggest he planned his attacks months in advance, gradually gathering the tools and the expertise he needed.

His diary entry on April 27 reads: "I made the order for the fertilizer which were to be delivered a week later. Prior to making this order I had officially registered my company as an agricultural entity."

He adds: "The last week in the capital I spent a lot of time with friends, partying and attending various social events. I knew that it would be the last chance for a very long time."

On July 4 he wrote: "Began the preparations for a trip to extract the armor cache I had dug down a year ago..."

Fin Larkin, a consultant psychiatrist familiar with making assessments of offenders' minds, said they "have to be pretty patently mentally unwell" to successfully claim insanity.

"It's not impossible...but it's actually quite difficult to fake," he said.

Larkin, who works in the personality disorder section of Britain's Broadmoor high-security psychiatric hospital, which has housed and treated some of its most notorious killers, said psychiatrists making such assessments take as their starting point that everything the defendant says is a lie.

"They've often got strong motivation not to tell the truth, they may be very well researched, they may have planned things for quite a while, they may be good mimics who have spent time with mentally ill people," he said.

Experts say that while there are some common factors that often crop up in the backgrounds of mass killers -- a difficult childhood, a lack of empathy, sadistic behavior and difficulties forming and maintaining relationships -- these on their own offer few clues for a mental illness diagnosis.

For a proper assessment, offenders should ideally be admitted to a high-security hospital where psychiatrists and other specialists monitor them day and night to observe their reactions to people, situations and the stresses of daily life.

This could and should take several months, Fazel said, and should be combined with a detailed search of the offender's life course -- including interviews with family, old friends, online contacts, analyses of diaries, and going back to school reports and earlier notes from doctors or other professionals.

"It's too early at the moment to say much about this man in terms of diagnosis, but it's very important to read his writings, interview informants, friends and family and get as much objective information as possible."