Witnesses on Thursday described in chilling detail how mass murderer Anders Behring Breivik tricked them into believing he was a policeman on the ferry to Utoya island, where he then killed 69 people — mostly teenagers — in a shooting spree.

Jon Olson, captain of the MS Thorbjoern ferry, told the Oslo District Court about his "angst and full panic" as he desperately tried to contact police about the attack on the island after his ferry docked at Utoya.

Breivik has admitted to a bombing in Oslo's government district that killed eight people and the subsequent shooting massacre at a Labor Party youth camp on Utoya. He claims the attacks were "necessary" and that the victims had betrayed Norway by embracing immigration.

Olson, who lost his partner, Monica Boesei — the second person to die in the shootings — said neither he nor his crew suspected the uniform-clad Breivik to be anything other than a police officer who had come to inform them about the Oslo attack. Breivik boarded the boat some two hours after setting off the bomb.

"I don't remember if I saw him shooting Monica, but I think I did," Olson calmly told the court about his partner, who worked on the island as a manager for over 20 years.

Breivik carried 1,000 rounds of ammunition

Breivik, dressed in a black suit and grey tie, listened impassively to witnesses and police giving evidence on the 11th day of the terror trial.

Investigating officers told the court how he shot his victims, beginning with a security official on the island and then 68 others, mostly the youth who were participating in the summer retreat.

Breivik's weapons included a Ruger Mini-14 Ranch rifle, equipped with a bayonet rifle, a pistol and more than 1,000 rounds of ammunition. He was also carrying a gas mask, a tourniquet, flashlight and three chocolate bars when he arrived on the island, police said.

He coldly described the attacks in gruesome detail during earlier court sessions.

Since he has admitted his actions, Breivik's mental state is the key issue for the trial to resolve. If found guilty and sane, Breivik would face 21 years in prison, although he can be held longer if deemed a danger to society. If declared insane, he would be committed to compulsory psychiatric care.

Breivik said last week that being declared insane would be the worst thing that could happen to him because it would "delegitimize" his views.