Studying 300 cases of people charged with jihadist terrorism in the United States since September 11, 2001, author Peter Bergen found the perpetrators were "generally motivated by a mix of factors", including "militant Islamist ideology; dislike of American foreign policy in the Muslim world; a need to attach themselves to an ideology or organization that gave them a sense of purpose"; and a "cognitive opening" to militant Islam that often was "precipitated by personal disappointment, like the death of a parent".