When the 9-11 Commission set out on their task the first decision they had to make to make was "where to start?" which had within it the implied question "who to believe?" They needed to establish a basic framework about the timeline of the attacks themselves, who the perpetrators were, what the activities of the killers were before 9-11. That framework shaped the questions they asked, the problems they identified, and helped screen out information of dubious reliability.
Captain Ed noted how oddly the screening worked on the Able Danger material when it was presented to the Commission staff:
Having heard this from two separate sources, one would expect that the Commission would insist on getting the data from the Pentagon themselves. When people call in tips to investigators on criminal cases, do police refuse to follow up because they didn't provide video and fingerprint evidence when they called? Kean says that the Commission requested the data three times from the Pentagon and didn't get the documentation they wanted.
If the original framework is solid, then the staff was doing the smart thing. They cannot chase down every outrageous claim, so they ask the witnesses to provide some evidence.
But that is a big if. What if the initial framework is weak? This post by Douglas Farah at the Counter-terrorism Blog points to an inherent problem:
It was this type of thinking that led Commission staff to ignore the overwhelming evidence of al Qaeda's involvement in the diamond trade as well. They received information from the prosecutor and investigators for the Special Court for Sierra Leone that they deemed unlikely because it was not fitting with what the Community, either out or ignorance or malice, was telling them. They requested information in writing from me, and said they had read the book. But they did not find me, the Special Court, Belgian police reports or other investigations to fit the conventional wisdom. Worse yet, they specifically declined to interview eye witnesses that the Special Court offered to make available to them, with no conditions or restrictions.
These are serious omissions, as is the omission of the Able Danger information. Not everything could be run down or every lead chased in the time allotted to the Commission for its work. The Bush administration compounded the problem by slowing the release of information and fighting the Commission on many unnecessary issues. But these were not issues where it would have been hard to gather more information and at least weigh in that such information exists.
At the same time, the Commission noted in its Monograph on Terror Financing that "Terrorist financing was not a priority for either domestic or foreign intelligence collection. As a result, intelligence reporting on the issue was episodic, insufficient and often inaccurate" (p. 4). The Monograph added that both the CIA and FBI failed to "comprehend al Qaeda's methods of raising, moving or storing money" (pp. 5-6). Yet the Commission staff chose to use only the information of institutions singled out for failure to understand terror finance in its report.
That is a real kick in the gut: "Yet the Commission staff chose to use only the information of institutions singled out for failure to understand terror finance in its report."
I think there is a long list of issues where the Commission anchored its framework on suspect sources. They used the FBI timeline for Atta's movements, they relied on the CIA for questions of terror financing, they talked to Clarke and Berger about the thwarting of the LAX bombing but never talked to the woman who arrested the guy with the explosives at the border.
UPDATE: Captain's Quarters has another example i should have mentioned-- the reliance on KSM for many of the details about the inner workings of the plot and al Qaeda. For example:
How good was the data for the Atta timeline, and how solid did the Commission nail down his movements? Looking at the data on pages 167 and 168 of the report, it appears that all of the information that the Commission used to establish travel timelines for the Atta cell came from interrogations of Ramzi Binalshibh and Khalid Sheikh Mohammed. These two AQ officers later also discounted Atta's travel to Prague in April 2001, despite the insistence of Czech intelligence that he met with the Iraqi envoy and an IIS agent at that time.
These guys might be the only game in town, but they are not fully cooperating with their interrogators. From the Commission report, p.514 note 4:
In an assessment of KSM’s reporting, the CIA concluded that protecting operatives in the United States appeared to be a “major part” of KSM’s resistance efforts. For example, in response to questions about U.S. zip codes found in his notebooks, KSM provided the less than satisfactory explanation that he was planning to use the zip codes to open new email accounts.