If people expected 9/11 to happen and wanted to profit from it, they could do so on the stock market. One way is to sell short the stocks of United Airlines and American Airlines. Selling short is borrowing stock from the stockbroker, selling it, and then hoping the price goes down so that you can buy it back for less and keep the change. For more leverage, one could buy put options. A put option is the right to sell a certain number of shares of a certain stock at a certain future date and price. For example, a put option to sell 100 shares of United Airlines stock at $50 per share on October 1, 2001.
In the days before 9/11 there was a huge increase in the number of put options purchased on these airlines, compared to the normal average daily volume. The CIA is said to monitor trends like this to predict political and economic events. This huge increase in the volume of put options suggests that the people buying these put options knew in advance that 9/11 was about to happen. This is an area of investigation that has never been pursued by the government. It was initially reported in the media, even the mainstream media (Wall Street Journal) but they dropped it too.