So, what was this? Was Sissoko acting as a mule for getting money out of the bank and transferring it to other places?
One way to launder money is to buy things after all. Luxury goods like food, wine and art are particularly good for this. If I need to give you $1000 with plausible deniability, then if I go to your restaurant and pay $1000 for a $50 meal, that does the job.
So, someone wanted to quietly move millions of dollars out of a bank, and then gave it to someone who possibly laundered it, and did things like running and airline and buying military helicopters (what other military hardware was he not caught buying?).
Then the US diplomatic system went to bat for him in court. Black magic or black budget?
(Edit: the spies angle is probably a bit of a push, but I think speculating he was laundering for a bigger fish isn't)