We still don't know much about the details of what he was supposed to have done. But apparently a lot of this related to exchange traded futures. These trades are recorded in the stock / futures exchange's computers and monitored by exchange officials - latest news is that exchange officials have warned SG something is wrong many weeks ago (so SG did not just suddenly find out about a big loss 10 days ago).
As tk83 says, other staff (SG's treasury dept) must have sent him lots of money to finance his trades over a long period of time. It's just unbelievable to say the guy acted alone.
