In the wake of the New York City bike path terror incident on Halloween, many are asking how authorities can stop extremists like the man who ran over innocent people with a truck in the name of IS. One way is an undercover FBI agent — or more specifically, an undercover Muslim agent like Tamer Elnoury. Still active in the FBI, Elnoury disguises his voice to tell us a vivid story about his book “American Radical,” and the tough job that he and his colleagues do. He also shares the way he moves between his real and undercover lives.