The bin Laden group are very good at identity theft and falsifying ID documents. So you open up several options accounts months ahead of time; do your trades the Friday/Monday before; 9/11 happens, you cash out, wire the money overseas, and disappear. When the SEC investigators finally get around to looking for you, they discover the account's mailing address is an abandoned building.
I'm thinking the closure of the stock exchanges after 9/11 due to all the damage may have been an unexpected development for bin Laden, in that it may have allowed investigators to notice the goings-on before the markets reopened and the terrorists had a chance to cash-out their winnings.