regarding fuel contamination:
I know of one incident in the USA in which a plane had contaminated fuel (airliner), on takeoff roll, just breaking ground after Vr, both engines quit. runway remaining was adequate for a quick landing. It could happen, but I don't think that is the case here.
if radio contact 11 minutes after takeoff was normal, the plane would likely be above 10,000 feet. even if all engines quit, the plane would glide long enough for another radio call.
a bomb in the cockpit, killing the pilots could account for a lack of further radio calls.
one must wonder about security procedures in africa.
however, my first feeling is wx.