An author cited in the Norway massacre suspect's rambling, 1,500-word manifesto has expressed regret that her writing might have served to inspire his rampage.
Gisele Littman, who writes under the name Bat Ye'or, said that since her books are in the public domain she had no control over who quoted her.
"Of course I regret if this man took inspiration from what I wrote or from what other writers wrote," she said. "As an insane person he should have been treated before, and I am greatly saddened for all the young innocents who tragically lost their lives, and for their families."
Breivik cites Ms Littman's work approvingly many times in the document he posted shortly before Friday's attacks.
Her 2005 book Eurabia: The Euro-Arab Axis explores the relationship between European and Muslim countries. The term Eurabia has been used by some to describe the growing influence of Islam in Europe, although Ms Littman contended that her arguments are more complex.
She said some Europeans harbour a suppressed fear of Islamic colonisation that might erupt into violence.
"I think there is a lot of frustration, for instance in France and England, and I think discussions in our society should be encouraged," said Ms Littman, who lives in Switzerland.
Referring to Breivik's attack, she added: "I'm afraid that this is something other people will imitate. I think the police must be very vigilant. This problem of Norway is a problem of Europe."
Ms Littman - whose next book is titled Europe, Globalisation And The Coming Universal Caliphate - is a Cairo-born Jew whose website says she fled the country in the 1950s after her nationality was revoked. She settled in England, where she married, before moving to Switzerland.