On Assignment

ON ASSIGNMENT: The Terrorist Next Door

Richard Engel tracks down the identities of Americans who left their country to fight for ISIS. Among thousands of documents obtained from a Syrian who claims to be an ISIS defector, NBC News found 15 names of Americans. For the last month, Richard Engel traveled across the country to find out why anyone would leave the U.S. to fight for one of its most dangerous enemies.

On Assignment: The Terrorist Next Door 12:58

Related: The 15 Who Left the United States to join ISIS

Behind the Scenes Photos from 'The Terrorist Next Door'
NBC News' Richard Engel interviews Abu Mohammed, who says he stole a drive containing the details of thousands of ISIS fighters ISIS files from a senior commander. NBC News
Recently retired FBI agent Gamal-Abdel Hafiz discusses with Richard Engel his thoughts on the Americans who have traveled overseas to join ISIS. NBC News
Richard Engel outside the home of the family of an American who traveled to Syria to join ISIS. NBC News