Some acts are just incomprehensible. Violent crime offends almost all people, but even as we condemn such acts, we can usually construct a plausible string of circumstances that explains such behavior and puts it in some context. Some crimes are inexplicable, beyond the imagination of all but the most twisted minds.

The murderous rampage by Mr. Anders Behring Breivik falls in the latter category. There is nothing that can explain the tragedy he has inflicted on Norway or cast it in anything except inhuman light. Mr. Breivik, 32, is a middle-class Norwegian who led a relatively quiet life — until last week. He joined the anti-immigrant Progress Party, Norway’s second biggest party over a decade ago, paying dues from 1999 to 2004.

Earlier this year, he moved from Oslo to the countryside, where he established an organic farm. It is now believed that he did that to gain access to large quantities of fertilizer that he could use to build a bomb.

On Friday, Mr. Breivik allegedly planted a car bomb in front of Norwegian government offices in downtown Oslo. The explosion killed eight people and wounded several dozen others; the casualties were light because the attack was late on a Friday during summer, when many people had left for the weekend or departed for summer vacation.

Next, he went to a youth camp run by the ruling Labor Party on an island 40 km from Oslo. The camp is an annual event for the next generation of Labor Party leaders and supporters. Mr. Breivik showed up in a police uniform and proceeded to shoot participants for over an hour. He killed 68 people at the youth camp before surrendering to the police.

The attack stunned Norway and the world. On a per capita basis, the death toll was even larger than the Sept. 11, 2001, terror attacks on the United States.

More remarkable — or horrific — was Mr. Breivik’s apparent calm and detachment. During the 90-minute rampage on the youth camp, Mr. Breivik would call children to him — he was dressed as a policeman — and then shoot them at point-blank range. When the police responded to the attack, Mr. Brievik surrendered to ensure that he would have his day in court and be able to make his case to the public.

In fact, he has already done that. Hours before he began his assault, he published a 1,500 page manifesto and a four-part video on the Internet in which he explained what had driven him to commit such atrocities.

According to his creed, Europe has been under assault by multiculturalism, “cultural Marxism” and “Islamization.” These first attacks — “atrocious, but necessary in his own head,” reported his lawyer — were needed to spread the word and launch a European-wide revolution led by the Knights Templar.

Targeting the youth camp would inflict the “greatest possible loss” on the ruling Labor Party, which ran the camp and was, according to Mr. Breivik, responsible for destroying Norwegian culture and importing Muslims en masse.

Killing was regrettable, and the deaths were to be minimized, but something had to be done to “save” Norway and Europe and it would be pathetic if the results had not been so bloody, according to his thinking.

The first question for law enforcement officials is whether Mr. Breivik acted alone. He originally said he acted alone, and then claimed that there were two other cells “in his organization.”

He fits the profile of a loner — distant from his parents, obsessed with video games, mostly unknown to others — but there is no mistaking the sad fact that his twisted view of multiculturalism is shared.

Mr. Breivik claims to have attended a meeting of like-minded people in London in 2002. The aim of the meeting was to reconstitute the Knights Templar Europe, a throwback to the Crusades, who would seek “to seize political and military control of Western European countries and implement a cultural conservative political agenda.”

The hysterical notion of a Muslim takeover of Europe is hard for thinking people to take seriously, but it enjoys wider currency than most of us care to admit. After all, many Americans believe that they too are under assault, as indicated by the campaigns against Shariah law or the complaints against the opening of mosques in U.S. communities.

It is revealing that many commentators reflexively believed that the blasts in Oslo were the work of Islamic terrorists.

In the aftermath of 9/11, moderate Muslim leaders were blamed for not repudiating the acts of extremists among their faith. Today, a similar burden falls on Christian leaders and the anti-immigration advocates to repudiate the acts of extremists.

Merely dismissing Mr. Breivik as deluded or a “psychopathic loner” is not a response. He is deluded, and the failure to condemn his actions for the crimes that they are as well as the political gibberish that he has used to justify those crimes will only encourage others to follow him.

The people of Norway have spoken up. In the aftermath of last week’s attacks, more than 100,000 of them rallied in Oslo and tens of thousands others joined hands elsewhere in the country to show their support for the government and the peaceful multiculturalism that Mr. Breivik considered a threat. Others should join them.

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