Islam expert Robert Spencer has been threatened with death for his new book, "Did Muhammad Exist?: An Inquiry into the Obscure Origins of Islam."

Spencer said the threats issued via the social networking site Twitter are part of the increasing pressure by Muslims to impose Islamic law – which bans criticism of Muhammad – on the free world.

Spencer, director of JihadWatch.org, told WND: "This is an increasing tactic on the left, as well as by Muslims. They don't seem to have any attachment to the principle of freedom of speech."

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