Excerpts from the report of the National Commission on Terrorist Attacks Upon the United States. The commission concluded that the attacks of Sept. 11, 2001, represented failures of policy, capabilities, management -- and imagination. Here is how the panel elaborated on the last factor:

Imagination is not a gift usually associated with bureaucracies. For example, before Pearl Harbor the U.S. government had excellent intelligence that a Japanese attack was coming, especially after peace talks stalemated at the end of November 1941. These were days, one historian notes, of "excruciating uncertainty." The most likely targets were judged to be in Southeast Asia. An attack was coming, "but officials were at a loss to know where the blow would fall or what more might be done to prevent it." In retrospect, available intercepts pointed to Japanese examination of Hawaii as a possible target. But, another historian observes, "in the face of a clear warning, alert measures bowed to routine." It is therefore crucial to find a way of routinizing, even bureaucratizing, the exercise of imagination. Doing so requires more than finding an expert who can imagine that aircraft could be used as weapons. Indeed, since al Qaeda and other groups had already used suicide vehicles, namely truck bombs, the leap to the use of other vehicles such as boats (the Cole attack) or planes is not far-fetched.

Yet these scenarios were slow to work their way into the thinking of aviation security experts. . . .

After the 1999-2000 millennium alerts, when the nation had relaxed, [Richard A.] Clarke held a meeting of his Counterterrorism Security Group devoted largely to the possibility of a possible airplane hijacking by al Qaeda. In his testimony, Clarke commented that he thought that warning about the possibility of a suicide hijacking would have been just one more speculative theory among many, hard to spot since the volume of warnings of "al Qaeda threats and other terrorist threats was in the tens of thousands -- probably hundreds of thousands." Yet the possibility was imaginable, and imagined. In early August 1999, the FAA's Civil Aviation Security intelligence office summarized the [Osama] bin Laden hijacking threat. After a solid recitation of all the information available on this topic, the paper identified a few principal scenarios, one of which was a "suicide hijacking operation." The FAA analysts judged such an operation unlikely, because "it does not offer an opportunity for dialogue to achieve the key goal of obtaining [Omar Abdel] Rahman and other key captive extremists. . . . A suicide hijacking is assessed to be an option of last resort."

Analysts could have shed some light on what kind of "opportunity for dialogue" al Qaeda desired. The CIA did not write any analytical assessments of possible hijacking scenarios.

One prescient pre-9/11 analysis of an aircraft plot was written by a Justice Department trial attorney. The attorney had taken an interest, apparently on his own initiative, in the legal issues that would be involved in shooting down a U.S. aircraft in such a situation.

The North American Aerospace Defense Command imagined the possible use of aircraft as weapons, too, and developed exercises to counter such a threat -- from planes coming to the United States from overseas, perhaps carrying a weapon of mass destruction. None of this speculation was based on actual intelligence of such a threat. . . . We can therefore establish that at least some government agencies were concerned about the hijacking danger and had speculated about various scenarios.

The challenge was to flesh out and test those scenarios, then figure out a way to turn a scenario into constructive action.

Since the Pearl Harbor attack of 1941, the intelligence community has devoted generations of effort to understanding the problem of forestalling a surprise attack. Rigorous analytic methods were developed, focused in particular on the Soviet Union, and several leading practitioners within the intelligence community discussed them with us. These methods have been articulated in many ways, but almost all seem to have at least four elements in common: (1) think about how surprise attacks might be launched; (2) identify telltale indicators connected to the most dangerous possibilities; (3) where feasible, collect intelligence on these indicators, and (4) adopt defenses to deflect the most dangerous possibilities or at least trigger an earlier warning.

After the end of the Gulf War, concerns about lack of warning led to a major study conducted for [Director of Central Intelligence] Robert Gates in 1992 that proposed several recommendations, among them strengthening the national intelligence officer for warning. We were told that these measures languished under Gates's successors.

Responsibility for warning related to a terrorist attack passed from the national intelligence officer for warning to the [CIA's Counterterrorist Center]. An Intelligence Community Counterterrorism Board had the responsibility to issue threat advisories. With the important exception of analysis of al Qaeda efforts in chemical, biological, radiological and nuclear weapons, we did not find evidence that the methods to avoid surprise attack that had been so laboriously developed over the years were regularly applied. . . .

Clarke told us that he was concerned about the danger posed by aircraft in the context of protecting the Atlanta Olympics of 1996, the White House complex and the 2001 G-8 summit in Genoa.

But he attributed his awareness more to Tom Clancy novels than to warnings from the intelligence community. He did not, or could not, press the government to work on the systemic issues of how to strengthen the layered security defenses to protect aircraft against hijackings or put the adequacy of air defenses against suicide hijackers on the national policy agenda.

The methods for detecting and then warning of surprise attack that the U.S. government had so painstakingly developed in the decades after Pearl Harbor did not fail; instead, they were not really tried. They were not employed to analyze the enemy that, as the twentieth century closed, was most likely to launch a surprise attack directly against the United States.

Ground Zero in Lower Manhattan nearly three years after the Sept. 11, 2001, attacks on the World Trade Center.