converted to Islam after high school and began to spread extremism over the Internet. He was arrested attempting to board a flight to Somalia to join the Islamic terrorist group al-Shabaab. U.S. citizens in Islamic terrorist organizations Since 2007, over 50 U.S. citizens and permanent residents have been arrested or charged in connection with attempts to join Islamic terrorist organizations abroad, including al-Qaeda in the Arabian Peninsula (AQAP) and al-Shabaab. In 2013 alone, 9 Americans are known to have joined or attempted to join foreign terrorist organizations. U.S. citizens inside al-Qaeda provide insider's knowledge of the United States. Adam Gadahn was an American convert who joined al-Qaeda in the late 1990s. He released English-language propaganda videos, but Gadahn lacked charisma and his voice was replaced by Anwar al-Awlaki. Awlaki was a U.S. citizen of Yemeni descent, killed on September 30, 2011 by a U.S. missile strike in Yemen. Awlaki had religious credentials Gadahn lacks and a "gently persuasive" style; "tens of thousands, maybe millions, have watched [Awlaki's] lectures on the Internet." Another key U.S. citizen in al-Qaeda's power structure was a man named Adnan Shukrijumah. Shukrijumah is believed to be the highest ranking American in al-Qaeda. He was born in Saudi Arabia, grew up in Trinidad, and moved to Florida as a teenager; he was a naturalized American citizen and left the United States in the spring of 2001. Shukrijumah was a mystery to authorities until he was identified by Najibullah Zazi after Zazi was arrested for a failed plot to bomb transportation targets around New York City. Zazi had traveled to Afghanistan to fight U.S. forces, but Shukrijumah convinced Zazi to return to the United States and plan an attack there. In May 2014, Florida-born convert Mohammed Abusalha conducted a deadly suicide bombing while fighting for Islamist extremists in Syria. In 2014, Troy Kastigar and Douglas McAuthur McCain, two Americans who had previously converted to Islam, traveled to Syria and Iraq in order to join the global jihad in Syria and Iraq on behalf of the Islamic terrorist group ISIL/ISIS/IS/Daesh, and were killed in battle. In 2015, Zulfi Hoxha traveled to Syria where he became a significant figure among the ranks of ISIS. Places for radicalization Prison The United States has the world's largest prison population and "prisons have long been places where extremist ideology and calls to violence could find a willing ear, and conditions are often conducive