From the Vancouver Sun:

In the days before the attacks, there was unusually heavy trading in airline and related stocks using a market tactic called a "put option" that essentially bets that a stock will decline in value. If you were Osama, buying puts would be a great way to boost the value of your investment portfolio.

And sure enough, unusually high numbers of put options were purchased in early September for the stocks of AMR Corp. and UAL Corp., the parents of American and United -- each of which had two planes hijacked. The U.S. government is now investigating suspicious trading in 38 companies directly affected by the events of Sept. 11.