Financial Times :
"One interesting question remains unanswered: what role did terrorist financing play in the build-up to the attacks of September 11 2001? There are two pieces of evidence on this point. First, during the days immediately prior to 9/11 the volume of put options, investments that pay off only when a stock declines in price, surged in the parent companies of American Airlines and United Airlines. According to a footnote in the 9/11 commission report, this unusual trading had no connection to terrorism.
Some people remain suspicious, and have pointed to a second piece of evidence: the dramatic spike in US dollars - particularly Dollars 100 bills - in circulation in August 2001, one of the three largest monthly increases in 50 years. This surge might have an innocent explanation. But it might also indicate that people with advance knowledge of the attacks were hurriedly withdrawing cash."
http://www.911blogger.com/2005/08/more-to-pre-911-put-options-high-100.html