This case is still surrounded with "confidentiality" which leads to may rumours.
The latest we heard is that the transponder of the HS 125 was known to have "problems" , and even the whole aircraft airworthiness was questioned.
A Crew that took a wrong clearance / wrong read back, and flew at the wrong ALT combined with a U/S transponder could explain the accident. But it opens a lot of questions which obviously both ASECNA and the Senegalese CAA have difficulties with .
@ Teddy Robinson : TCAS is like Tango : you need to be 2 for it to work.