DEVASTATED BY the worst attack it has suffered since World War II, Norway will spend a long time mourning the government workers and promising young people tragically murdered last Friday and reflecting on how a killer apparently acting alone could have committed so much mayhem without being stopped. Civilized people everywhere will share the pain of such a terrible and senseless loss. Inevitably, too, there will be debate about who, besides confessed killer Anders Behring Breivik, bears responsibility for his hideous acts. There, some caution would be helpful — not that we’ve seen it so far.

In the hours after the bombing and shooting massacre Friday numerous commentators prematurely rushed to the conclusion that Muslim extremists were to blame. There was considerable speculation about why Norway would have been targeted: its participation in NATO’s Afghanistan mission? Its prosecution of an extremist Iraqi Kurdish cleric? Once Mr. Breivik was revealed to be a right-wing anti-Muslim extremist and self-styled crusader, the rush began to tar right-wing European political parties and “counter-jihad” Web sites in the United States with his evil.

So it seems worth underlining that, to date, there is no evidence that Mr. Breivik collaborated with anyone — and plenty that he is a deeply deranged individual. The 1,500-page treatise he published on the Internet contains many quotations from such Web sites as the U.S.-based Jihad Watch — but also large borrowings from the writings of Unabomber Theodore J. Kaczynski, another disturbed loner. According to Norwegian press accounts, Mr. Breivik once belonged to Norway’s Progress Party but dropped out years ago because he considered it far too moderate.

It’s true that far-right political parties appealing to anti-Muslim sentiment have grown rapidly in Europe, particularly in Scandinavian countries such as Denmark, Sweden and Finland. It’s a disturbing development, but it is happening in part because some of those parties are trying harder to appeal to mainstream voters and because their attacks on “multiculturalism,” as practiced in Western Europe, resonate widely. It’s no accident that German Chancellor Angela Merkel and British Prime Minister David Cameron have joined in the critique.

It may be true, as some experts are saying, that law enforcement agencies have not paid enough attention to the threat of violence from anti-Muslim extremists in the United States and Europe. If so, the tragic events in Norway are likely to lead to renewed scrutiny of that danger, as they should. We have no sympathy for populists who appeal to anti-Muslim bigotry on either side of the Atlantic. But just as al-Qaeda — and not Islam — was responsible for the attacks of Sept. 11, 2001, it is Mr. Breivik who should be held accountable for the Norway massacre.