Speaking as an amateur in all aspects of this (esp forensic security trading analysis), I find Poteshman's article more compelling than the official government verdict. For unknown reason I cannot copy from the pdf of 9-11 Commissions report, but you can see it on page 499, note 130 to Chapter Five: "The SEC and the FBI, aided by other agencies and the securities industry, devoted enormous resources to investigating this issue ... These investigators have found the apparently suspicious consistently proved innocuous."

You can see their full explanation (one paragraph!) here: Report of the National Commission on Terrorist Attacks upon the United States (you can scroll down to download just the notes section, as the full report is 7.4 meg).