Breivik, a self-styled anti-Muslim militant, looked pleased as Judge Wenche Elisabeth Arntzen read the ruling, declaring him sane enough to be held criminally responsible for Norway’s worst peacetime attacks.

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Mass murderer Anders Behring Breivik, makes a salute after he arrives at the courthouse in Oslo today. AP

The judgment could mark the end of a legal process that has haunted Norway in the 13 months since Breivik’s rampage on July 22, 2011. His lawyers said before the ruling that he would accept a prison term, and only appeal if ruled insane.

Going against the recommendation of prosecutors, who had asked for an insanity ruling, the five-judge panel in the Oslo district court convicted Breivik of terrorism and premeditated murder. They imposed a sentence of “preventive detention,” a special prison term for criminals considered dangerous to society. Prosecutors have not said if they would appeal.

The judges set the minimum length of imprisonment to 10 years and the maximum at 21 years, the longest allowed under Norwegian law.

However, such sentences can be extended as long as an inmate is considered too dangerous to be released. Legal experts have said that in Breivik’s case that could mean he will spend the rest of his life in prison.

Some who lost loved ones in the attacks welcomed the ruling.

“Now we won’t hear about him for quite a while. Now we can have peace and quiet,” Per Balch Soerensen, whose daughter was among those killed in the shooting massacre, told Denmarks TV2. “He doesn’t mean anything to me, he is just air.”

Wearing a dark suit and sporting a thin beard, Breivik smirked as he walked in to the courtroom to hear his sentence and raised a clenched-fist salute.

Breivik, 33, confessed to the attacks during the trial, describing in gruesome detail how he detonated a car bomb at the government headquarters in Oslo and then opened fire at the annual summer camp of the governing Labor Party’s youth wing. Eight people were killed and more than 200 injured by the explosion. Sixty-nine people — mostly teenagers — were killed in the shooting massacre on Utoya island. The youngest victim was 14.

During the trial, Breivik said that being sent to an insane asylum would be the worst thing that could happen to him and accused Norwegian authorities of trying to cast him as sick to deflate his political views. His lawyers say Breivik is already at work writing sequels to the 1,500-page manifesto he released on the Internet before the attacks.