In the days leading up to 9/11, analysis shows a rise in the put to call ratio for United Airlines and American Airlines, the two airlines from which planes were hijacked on 9/11. Between September 6 and 7, the Chicago Board Options Exchange recorded purchases of 4,744 "put" option contracts in UAL and 396 call options. On September 10, more trading in Chicago saw the purchase of 4,516 put options in American Airlines, the other airline involved in the hijackings, with a mere 748 call options in American purchased that day. No other airline companies had an unusual put to call ratio in the days leading up to the attacks. The 9/11 Commission concluded that all these abnormal patterns in trading were coincidental.