The timing of the Sept. 11 terrorist plot was delayed at least once. One of the plot's pilots appears to have considered pulling out at the last moment. Throughout the summer of 2001, top Al Qaeda leaders bickered among themselves about the merits of striking within the United States.
The conspiracy that eventually pulled off the deadliest attacks ever on US soil was far from a collection of evil geniuses. The plot was brittle and would not have been impossible to disrupt, according to findings of the National Commission on Terrorist Attacks Upon the United States.
The implication: For all their flexibility and resilience, Al Qaeda cells are vulnerable. It is not necessarily inevitable that a tragedy on the scale of Sept. 11 will soon happen again.
"These are just people. It is important for us to reduce this to that," says a source close to the commission. "These are just people who have problems. They are not infallible."
As it nears the end of its investigations, the 9/11 commission has produced perhaps the most authoritative account of Osama bin Laden's terror network and its US operations yet made public.
The terror training infrastructure that Mr. bin Laden established in Afghanistan and elsewhere was "quite good," judges a commission staff report. Trainees were encouraged to think creatively about ways to commit mass murder.
Among their ideas were taking over a Russian launcher and forcing its personnel to fire a nuclear missile at the US, dispensing poison gas into the air conditioning of a targeted building, and hijacking an airliner and crashing it into an airport or nearby city.
That last idea, of course, was the one that was adopted. It appears to have originated with Khalid Sheikh Mohammed, a Kuwaiti who claims to have been a radical jihadist since age 16 and who provided some of the money that financed the 1993 World Trade Center bombing.
"In early 1999, bin Laden summoned [Mohammed] to Kandahar to tell him that his proposal to use aircraft as weapons now had al Qaeda's full support," says a commission report.
The details of how that plot was carried out - how operatives filtered into the US, took flight training, and took training flights cross-country - are fairly well known. But according to details provided by the commission, it was far from a precision operation, despite the tragedy of its ultimate success.
To begin with, the operation was quickly scaled back from Mohammed's grandiose vision. He had wanted 10 aircraft involved, with nine meant to hit targets on the US coasts. He himself would fly the 10th plane - but rather than crashing it, he would kill all males on board, land, and make a speech denouncing US policies in the Middle East before releasing all women and children.
Then the operation was split into two parts - the US half, and a Southeast Asian version intended to explode airliners in the air. Then coordination difficulties caused the second half to be canceled.
Setting a date for the attacks became a problem. Bin Laden kept pressuring the plotters to move as early as possible.
There were personnel problems as well. Two of the earliest participants, Nawaf al-Hazmi and Khalid al Mihdar, were supposed to train in the US as pilots, but their poor English and general unfamiliarity with Western culture prevented it. They were shunted to the category of "muscle," and other pilot trainees were brought in.
In addition, the 9/11 commission believes that one of the eventual pilots, Ziad Jarrah, was torn about participating and nearly backed out. He kept flying back to Europe and the Middle East to visit his girlfriend and his family. Another Al Qaeda operative, Zacarias Moussaoui, may have been training to take Mr. Jarrah's place - though Mr. Moussaoui's suspicious behavior resulted in his arrest before the Sept. 11 attacks.
At least nine other hijacker candidates were dropped from the plot, claims the 9/11 commission. They either failed to obtain US visas, withdrew under pressure from their families, or were scratched by the Al Qaeda leadership for unknown reasons. "Internal disagreement among the 9/11 plotters may have posed the greatest potential vulnerability for the plot," says a commission report.
The fact is the plot was indeed vulnerable, say some experts. A natural tendency exists to think of one's adversaries as perfect if they are shadowy and have already struck. "The reality is the terrorists, though violent, are human beings," says Jim Walsh of Harvard's John F. Kennedy School of Government. "Some are competent; some are incompetent."
But the next plot may look nothing like the one that has already been carried out. That's one of law enforcement's biggest challenges. The patterns of the past may be no help in protecting the future.
"If you go back through all of Al Qaeda's successful actions, each has been different and meticulously planned," notes Robert Pfaltzgraff, a security expert at Tufts University's Fletcher School.