Its operator, who describes himself as a Virginia consultant and uses the pseudonym “Baron Bodissey,” wrote on the site Sunday that “at no time has any part of the Counterjihad advocated violence.”

The name of that Web site — a reference to the siege of Vienna in 1683 by Muslim fighters who, the blog says in its headnote, “seemed poised to overrun Christian Europe” — was echoed in the title Mr. Breivik chose for his manifesto: “2083: A European Declaration of Independence.” He chose that year, the 400th anniversary of the siege, as the target for the triumph of Christian forces in the European civil war he called for to drive out Islamic influence.

Marc Sageman, a former C.I.A. officer and a consultant on terrorism, said it would be unfair to attribute Mr. Breivik’s violence to the writers who helped shape his world view. But at the same time, he said the counterjihad writers do argue that the fundamentalist Salafi branch of Islam “is the infrastructure from which Al Qaeda emerged. Well, they and their writings are the infrastructure from which Breivik emerged.”

“This rhetoric,” he added, “is not cost-free.”

Dr. Sageman, who is also a forensic psychiatrist, said he saw no overt signs of mental illness in Mr. Breivik’s writings. He said Mr. Breivik bears some resemblance to Ted Kaczynski, the Unabomber, who also spent years on a manifesto and carried out his mail bombings in part to gain attention for his theories. One obvious difference, Dr. Sageman said, is that Mr. Kaczynski was a loner who spent years in a rustic Montana cabin, while Mr. Breivik appears to have been quite social.

Mr. Breivik’s declaration did not name Mr. Kaczynski or acknowledge the numerous passages copied from the Unabomber’s 1995 manifesto, in which the Norwegian substituted “multiculturalists” or “cultural Marxists” for Mr. Kaczynski’s “leftists” and made other small wording changes.

By contrast, he quoted the American and European counterjihad writers by name, notably Mr. Spencer, author of 10 books, including “Islam Unveiled” and “The Truth About Muhammad.”