Sept. 11 began and ended with questions — questions that started while the attack was in progress (How many planes?) and multiplied as the towers fell (How many dead?), questions asked out loud to no one in particular or posed silently to television screens (How could it happen?).
Since the attacks, even as the United States launched a wide-ranging "war on terrorism" overseas and created a sprawling Homeland Security complex at home, an equally intense effort by victims' kin and some investigators has hunted answers.
For Saturday's third anniversary of the attacks, some of the questions have been satisfied by the recent final report of the Sept. 11 commission.
Other mysteries remain. Some concern whether the necessary changes are being made to improve U.S. terrorism defenses. Others pertain to intelligence information that is still kept classified by the government. A few concern details of pure historical interest.
The most vexing questions, and the least answerable, deal with exactly what happened on Sept. 11 at the top of the towers and in the four doomed planes. These are the facts known only to those who perished, the victims and killers.
"There are still some unanswered questions because obviously the people who were at the heart of the plot are dead," the commission chairman, former New Jersey Gov. Thomas Kean, said when releasing the report, which he called "the definitive work" on the attacks. "If we capture Osama bin Laden – when we capture Osama bin Laden, I hope – and he answers questions, there may be new information."
Bin Laden may know, for example, whether the target for the fourth plane was the Capitol or the White House. That detail may seem trivial, but it completes the story of what the passengers of Flight 93 accomplished by forcing that plane down in Pennsylvania.
It also could help address one of the unknowns in the Sept. 11 report — whether the military would have been capable of shooting down Flight 93 if it approached Washington.
"NORAD officials have maintained that they would have intercepted and shot down United 93," the report reads. "We are not so sure."
The circumstances of the shootdown order itself are unclear. Some witnesses recall hearing Vice President Dick Cheney getting authorization from President Bush to tell military pilots to shoot down suspicious aircraft; other witnesses do not record that conversation. Also unclear is how the shootdown order was transmitted through NORAD.
Other questions concern the plot and plotters, such as whether the 19 hijackers had a support network in the United States as they prepared their scheme. The Sept. 11 report offers no conclusions on this count, but does cast doubt on the notion.
"The evidence is thin — simply not there for some cases, more worrisome in others," it reads.
Also unknown is why hijackers Nawaf al Hazmi and Khalid al Mihdhar traveled to California in 2000, the identity the man named "Khallam" with whom Hamzi and Midhar met or why hijackers Mohammed Atta and Abdul Aziz Omari went to Portland, Maine on the eve of the attacks. Because there were no surveillance cameras, no one knows for sure if the hijackers who boarded plans in Boston or Newark set off metal detectors or were searched.
As for the biggest question of all — whether the attacks could have been prevented — the Sept. 11 panel made no ruling.
"I can't say we are sure," Kean said. "We do not know. We think it's possible, but we have not drawn that absolute conclusion because we don't believe that absolute conclusion is justified by the facts."
At the World Trade Center, there is no certainty as to when people in the North Tower were told to evacuate, or if emergency phones were operating.
It is unclear why tenants in the South Tower were initially told to return to their offices after the other tower was hit. That puzzle may never be solved, because the person who made that announcement, like so many witnesses to the most graphic details of the day, is dead.
Dead too are the people who could tell exactly how the hijackers gained entry to the cockpits of the planes they seized — or the other specifics of the last moments of the hundreds of passengers and crew.
"There is information that is unknown, that we will never know, that will always be a subject of speculation for me, and regretfully, I will always think the worse," said Donald Goodrich of Bennington, Vt., whose son Peter died aboard United Flight 175, which struck the World Trade Center's South Tower.
"It's hard for me to stop imagining the images that were unfolding before my son's eyes in his final minutes, and putting them in a horrific light. I try not to but it's very hard not to, and because of the pure absence of information those speculative and, in an intellectual sense, admittedly unreasonable images will continue to haunt me."
Much is also unknown about the fate of victims at the World Trade Center. Of the firefighters who died in the North Tower, some never heard the evacuation orders issued after the South Tower collapsed. Others might have heard the order and ignored it, perhaps to assist an injured civilian. Some rescuers may have reacted to the order but simply did not get out in time.
Among civilians, it cannot be known for sure how many died when the planes hit the towers. Reports of what happened on the floors above the crash area are incomplete, based largely on phone calls from people who died there.
That record, while patchy, is enough for some victims' relatives.
"I have a pretty clear idea of that picture," said Connie Taylor of Westport, Conn., whose son Brad Vadas, 37, died in the South Tower. "The 9/11 commission helped a little. They described it pretty clearly, that plus communication we had at the time (from Vadas). That's pretty clear to me — pretty sad and pretty clear."
For all the details that will never be known, there are some lingering questions that could ultimately be answered.
Goodrich hopes to eventually learn about information, now classified, pertaining to "the hijackers, their associates, their funding sources, the means by which they gained entry into the country, the nature of the weapons that were available to them and how they were able to get past security."
"Of course, there is an insatiable desire on my part to have this information," Goodrich said. "I want to know what is known."
And many family members are focused on the question of what has been done to shore up defenses against terrorism, and what will be done going forward.
"I'm mostly concerned with what has happened – what really concrete changes have been made to make our world a safer place," said Taylor, mentioning the confusion among air traffic controllers, the scarcity of fighter planes to track suspicious jets and the communications problems that affected New York City cops and firefighters. She has heard of little progress.
"It just smacks of everything that seems to go so slowly with our government," Taylor said. Goodrich agrees.
"There is no reason other than institutional inertia and self-interest that will stand in the way of making changes which are broad in their scope and dramatic in their effect to protect the public," he added.
By Jarrett Murphy
By Jarrett Murphy