Sept. 8, Flight 91 was ready to take off from Newark, N.J., for Dubai, United Arab Emirates with a final destination of Kabul, Afghanistan, when agents of the F.B.I. boarded. The agents escorted 7 passengers off the plane and into a room where they were interrogated for 6 hours. The group, part of a tour organized by San Francisco-based Global Exchange to visit Afghanistan, missed their plane. A.C. Thompson reports.