What we know of the planning of 9/11 is largely derived from the CIA interviews of  two captured plotters. The 9/11 Commission was not allowed to interview them or their CIA interrogators and no in depth investigation to fill in the gaps in their stories was undertaken. Edward Jay Epstein, who has closely watched this process, notes the Spanish have made a very strong case for a far-less contained plot, one in which Atta's cohorts in Spain and elsewhere in the world played a key role:

Yet if Mr. Garzon is correct about the Spanish connection to 9/11, it is not only the effectiveness of the CIA's interrogation of its al Qaeda prisoners that is called into question. The information from Binalshibh, KSM and other detainees was used to fill in the missing pieces of the jigsaw, and those gaps concerned the contacts the 9/11 conspirators might have had with others wishing to harm America. By saying that no one else was involved--not in Spain, Iran, Hezbollah, Malaysia, Iraq, the Czech Republic or Pakistan--these detainees allowed the 9/11 Commission to complete its picture of al Qaeda as a solitary entity.

Yet to come to its conclusion on this most fundamental issue, the commission was prohibited from seeing any of the detainees whose accounts it relied on. Nor was it allowed even to question the CIA interrogators to determine the way that information was obtained. The commission's joint chairmen themselves later acknowledged that they "had no way of evaluating the credibility of detainee information." So when Judge Garzon comes up with evidence that runs counter to detainees' claims, cracks begin to emerge in the entire picture,