Complex sentence: It is possible that, unlike the other hijacked flights, Burlingame and Charlebois weren't stabbed to death and forced to the back of the plane due to a phone call by Barbara Olson, when she asked what she should tell the "captain".
Split into simpler sentences: It is possible that, unlike the other hijacked flights, Burlingame and Charlebois weren't stabbed to death and forced to the back of the plane. This could be due to a phone call by Barbara Olson, when she asked what she should tell the "captain".