OSLO, Norway — Forensic scientists have criticized a report that says confessed mass killer Anders Behring Breivik was sane when he massacred 77 people last year in a bomb attack and shooting spree.

In a letter published Friday, the Norwegian Board of Forensic Medicine questioned why that report had not taken into account "important information" provided by the killer's mother, who said that her son's behavior changed dramatically a year before the July 22 attacks.

Meanwhile, experts testifying in court threw doubt on Breivik's claim that his lethal rampage was a planned suicide attack.

Breivik's mental state is the key issue of his ongoing trial and will determine his sentencing.

If found guilty and criminally sane, he would face 21 years in prison, though he could be held longer if deemed dangerous to society. If declared insane, he would be committed to compulsory psychiatric care.

Two psychological examinations carried out before Breivik's terror and murder trial started in mid-April reached opposite conclusions on the 33-year-old's sanity.

Information provided by the mother was central to the first report, which found Breivik psychotic at the time of his rampage. The second report declared him sane, but said he suffers from personality disturbances.

Judges had given the forensic board until June 1 to take a stand on the validity of the second psychiatrist's report. After receiving the letter, Judge Wenche Elizabeth Arntzen said Friday the court would focus on the concerns raised in the reports when questioning psychiatrists later in the trial.

In the attacks, Breivik set off a car bomb outside government buildings in central Oslo, killing eight people, and then went on a shooting rampage at a Labor Party youth retreat on Utoya island that left 69 dead.

He has admitted to the killings, but denies criminal guilt, saying the victims had betrayed their country by embracing immigration. He also insists he is sane. Last week, he declared that he would not appeal a guilty verdict if the court deems him sane.

A terror expert told the court on Friday that Breivik's killings did not conform to conventional suicide attacks.

"(They) lack many elements of a suicide attack. He did not make a martyr video, he did not die and had no bomb belt," said Brynjar Lia, from the Norwegian Defense Research Establishment. Lia also noted that Breivik called officials twice to surrender during the Utoya attack.

Early in the trial, Breivik said he had expected to die and rejected suggestions that he suffered from a narcissistic personality disorder.

"July 22 wasn't about me. July 22 was a suicide attack. I wasn't expecting to survive that day," he told the court on April 17. "A narcissist would never have given his life for anyone or anything."

Local media reported that prison authorities are preparing to keep Breivik behind bars for a long time. Although he will be kept apart from other prisoners because he's considered a high-risk inmate, prison officials plan to form a specially trained group, including guards, who will be allowed to visit him and provide him some company.

Knut Bjarkeid, the head of Ila prison where Breivik is being kept, told the Norwegian VG daily that their greatest concern is a hostage situation.

"Many of our efforts concerning Breivik are being carried out to prevent him from taking hostages which would be his only possibility of breaking through the layers of security measures," VG quoted him as saying.

Earlier this week, medical experts confirmed in court that Breivik likely was under the influence of stimulants at the time of the attacks after consuming a mixture that included ephedrine, caffeine and aspirin.

The trial is scheduled to last until June 22.

The Charges
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Anders Behring Breivik, a right-wing extremist who confessed to a bombing and mass shooting that killed 77 people on July 22, 2011, arrives for a detention hearing at a court in Oslo, Norway, Monday, Feb. 6, 2012. (AP Photo/Heiko Junge, Scanpix Norway)

Breivik is charged with terrorism and premeditated murder for a bombing in Oslo's government district, killing eight, and a shooting attack at a political youth camp, killing 69. He admits to the attacks but rejects criminal guilt. If convicted he would face a maximum sentence of 21 years in prison, though sentences can be extended if a criminal is considered a menace to society. If declared insane by the court, he would be committed to psychiatric care. Both sides can appeal the ruling to a higher court.