When U.S. authorities got their hands on terrorist Mohammed Mansour Jabarah in May 2002, he agreed to inform on some of the most influential al-Qaeda leaders. So instead of being sent to Guantanamo Bay, Cuba, or a high-security CIA detention facility, Jabarah was housed with relatively lax security at Fort Dix, N.J., where he was allowed to watch television and movies, speak to his family in Canada by telephone, go for walks and even make his own meals, all under 24-hour FBI watch. That arrangement soon proved to be a major problem for the bureau. In court papers filed in...