Insider trading before 9-11

Highly publicized allegations of insider trading in advance of 9/11 generally rest on reports of unusual trading activity in companies whose stock plummeted after the attacks.
In the month prior to the 11 September 2001 attacks on the World Trade Center and the Pentagon, unusual trading activity involving American and United Airlines stock was noted by market analysts, who at the time had no idea what to make of it. Wildly unusual discrepancies in the put and call ratio — 25 to 100 times normal — were reportedly observed in stock options of the two airlines. In one case, Bloomberg's Trade Book electronic trading system identified option volume in UAL (parent of United Airlines) on 16 August 2001 that was 36 times higher than usual.