Norwegian police have said they are confident that confessed mass killer Anders Behring Breivik acted on his own in terror attacks last year that killed 77 people and found no evidence that he belonged to a Europe-wide anti-Muslim network.

Three officers who testified at Breivik's trial told the court they found no proof of anyone being complicit in Breivik's plans or helping him in any way in the July 22 bomb attack in central Oslo and the subsequent shooting spree at a youth camp on an island.

Chief investigator Kenneth Wilberg said police were sure of their findings and that they also found no proof that Breivik belonged to Knights Templar, a far-right network, as claimed by the killer.

"Nothing in our investigation supports that Knights Templar exists," he said.

After Breivik surrendered on the island of Utoya where 69 people were massacred, he told police he was a resistance fighter in a militant group modelled after the Knights Templar - a Christian order that fought during the crusades.

But during the months-long investigation, officers have said they found no trace of the alleged anti-Islamic group.

Wilberg said 50 police officers were still investigating Breivik's attacks but that they did not expect to find new evidence that would change their conclusions.

The 33-year-old extremist has confessed to the July 22 massacre but denies criminal guilt, saying the victims had betrayed their country by embracing immigration.

Breivik's mental state is the key issue to be resolved during the trial.

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.

Last week, Breivik told the court that he would not appeal the verdict if the court deems him sane. Two psychological examinations carried out before the start of trial reached opposite conclusions on whether he is psychotic.

The trial is scheduled to last until the end of June.

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