There are many claims that some large investors had foreknowledge of the September 11, 2001 terrorist attacks, and used that to buy "put" options - especially in the affected airlines - that allowed them to benefit from stock drops.
Further, some people say that millions of dollars of profit from those transactions were never claimed.
Here are some examples:
CBS News, September 19, 2001:
Sources tell CBS News that the afternoon before the attack, alarm bells were sounding over unusual trading in the U.S. stock options market.
An extraordinary number of trades were betting that American Airlines stock price would fall.
[This article does not say the profits were left unclaimed.]
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This forum user quotes the 9/11 Commission Report investigation into insider trading, and the rejects the arguments made:
Therefore, as this turned out to be only one company, how could such a claim be made that said trading is referred to a general fall of airline-options and a general recommendation made by a trading newsletter? That fact that 95% of the put options were purchased by one investor is proof that this options-trade wasn't result of general business. This investor had a very special interest in these put-options.
It goes on to name the bank making 95% of the purchases of United Airlines put options as Alex Brown Inc.
[This article does not say the profits were left unclaimed.]
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This article takes a broader scope beyond just a couple of airlines, and says some of the profits were unclaimed.
The fact that $2.5 million of the put options remained unclaimed is not explained at all by market pessimism, and is evidence that the put option purchasers were part of a criminal conspiracy.
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Snopes refutes the claim:
The National Commission on Terrorist Attacks Upon the United States (also known as the "9/11 Commission") investigated these rumors and found that although some unusual (and initially seemingly suspicious) trading activity did occur in the days prior to September 11, it was all coincidentally innocuous and not the result of insider trading by parties with foreknowledge of the 9/11 attacks
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This article walks through the claims and suggests that there are non-sinister explanations for the trading and even the unclaimed money.