Anders Behring Breivik's killing spree was inspired by the ramblings of Unabomber Ted Kaczynski, it has emerged.
Part of Breivik's 1,500 manifesto was taken almost word for word from the first few pages of the anti-technology polemic written by Kaczynski.
The so-called Unabomber is in federal prison in the US for mail bombs that killed three people and injured 23 others across America from the 1970s to the 1990s.
Breivik changed a Kaczynski screed on leftism and what he considered to be leftists' "feelings of inferiority" -- mainly by substituting the words "multiculturalism" or "cultural Marxism" for "leftism".
For instance, Kaczynski wrote: "One of the most widespread manifestations of the craziness of our world is leftism, so a discussion of the psychology of leftism can serve as an introduction to the discussion of the problems of modern society in general."
Breivik's manifesto reads: "One of the most widespread manifestations of the craziness of our world is multiculturalism, so a discussion of the psychology of multiculturalists can serve as an introduction to the discussion of the problems of Western Europe in general."
The 32-year-old also cited an incident during the first Gulf War, when a Muslim friend cheered at reports of missile attacks against American forces, as a life-changing moment.
"I was completely ignorant at the time and apolitical but his total lack of respect for my culture (and Western culture in general) actually sparked my interest and passion for it," the suspect in Norway's bombing and mass shooting wrote.
But it was the NATO bombing of Serbia in 1999 that "tipped the scales" for him because he sympathised with Serbia's crackdown on ethnic Albanian Muslims in Kosovo.
A year later he said he realised that what he called the "Islamisation of Europe" couldn't be stopped by peaceful means.
Breivik's manifesto chronicled events that deepened his contempt for Muslims and "Marxists" he blamed for making Europe multicultural.
He suggested his friends didn't even know what he was up to, and comments from several people who had contact with the quiet blond man indicate he was right.
Jack Levin, a professor at Northeastern University who has written a number of books on mass murderers, said the manifesto helps Breivik show himself as more human.
"It makes the killer look like a victim rather than a villain," Levin said.
From September 2009 through October 2010, Breivik posted more than 70 times on Dokument.no, a Norwegian site with critical views on Islam and immigration.
In one comment, he entertained the idea of a European Tea Party movement.
In December of 2009, Breivik showed up at a meeting organised by the website's staff.
"He was a bit strange. As one could see from his postings, he had obviously read a lot but not really digesting it," said Hans Rustad, the editor of the website.
But Rustad said he "hadn't the faintest idea" about Breivik's murderous plans.
"Other people have the same views on the net and they don't go out and become mass murderers. So how can you tell?" Rustad said.
In the document, Breivik styles himself as a Christian conservative, patriot and nationalist. He looks down on neo-Nazis as "underprivileged racist skinheads with a short temper".
Breivik called his upbringing in a middle-class home in Oslo privileged even though his parents divorced when he was one and he lost contact with his father in his teens.
His parents split when the family lived in London, where his father, Jens Breivik, was a diplomat at the Norwegian Embassy in London.
A spokesman for the embassy, Stein Iversen, confirmed that Jens Breivik was employed at the embassy in the late 1970s, but wouldn't discuss his relationship with the Oslo suspect. Breivik said both parents supported Norway's centre-left Labour Party, which he viewed as infiltrated by Marxists.
His mother won a custody battle, but Breivik said he regularly visited his father and his new wife in France, where they lived, until his father cut off contact when Breivik was 15.
The father told Norwegian newspaper VG that they lost touch in 1995, but that it was his son who wanted to cut off contact.
"We've never lived together, but we had some contact in his childhood," the older Breivik, who VG said is now retired in France, was quoted as saying. "When he was young he was an ordinary boy, but reclusive. He wasn't interested in politics at the time."
He learned about Breivik's massacre on the internet.
"I was reading online newspapers and then I suddenly saw his name and picture on the net," he told VG. "It was a shock to find out. I haven't gotten over it yet."
Breivik's mother lives in an ivy-covered brick apartment building in western Oslo, currently protected by police.
Neighbours said they hadn't seen her since a few days before the shooting. Police said they've spoken to her and that she didn't know of her son's plans.
In his manifesto, Breivik said he had no negative experiences from his childhood, though he had issues with his mother being a "moderate feminist".
"I do not approve of the super-liberal, matriarchal upbringing though as it completely lacked discipline and has contributed to feminise me to a certain degree," he said.