Five years after the deadliest enemy attack ever on the United States, questions linger about the terror plot that toppled New York's World Trade Center, damaged the Pentagon and ignited the ongoing war on terrorism.
The killing of nearly 3,000 people on September 11, 2001, is not a "cold case."
But it also is not a closed case, and despite the huge body of evidence collected, unanswered questions remain. Among them:
•Who did the airliner hijackers meet as they spent months in the U.S. planning -- and traveling to places such as Las Vegas and Los Angeles?
•What was the ultimate source of money for the terrorist operation?
•What was the Washington target of United Flight 93, which crashed near Shanksville, Pa.?
•What motivated 19 relatively well-off Arab men, all Muslims, to become such horrific suicide bombers?
Lee Hamilton, vice chairman of the national September 11 commission, says motivation was the "big question" the panel could not answer.
"We were never able to answer why the 19 were willing to kill themselves. What was the motivation? I am talking in a very personal way," Mr. Hamilton, a Democrat who represented Indiana's 9th District in the House for 34 years, said in an interview wih The Washington Times.
"What was the motivation for each one of these hijackers? I think the question of motivation was opaque to us. We just could not nail it. They were dead, of course."
He said investigators guessed at a religious motive or a political grievance against the West, or more specifically the United States.
"Why do they hate us? It's one of the fundamental questions raised again and again," Mr. Hamilton said. "We couldn't pin it down for the 19."
The attack spurred the FBI to open the largest criminal investigation in its history.
Penttbomb, as the probe was code-named, culled the depths of radical Islam and the al Qaeda network to construct an authoritative chronology of the terrorists' four years of planning to drive fuel-packed jetliners into the World Trade Center, the Pentagon and an unidentified Washington symbol of political power.
The FBI followed more than a half-million leads and conducted more than 165,000 interviews.
The national September 11 commission, with former New Jersey Gov. Thomas H. Kean, a Republican, as chairman, examined the FBI's work and coaxed huge amounts of documents from the nation's intelligence agencies.
The commission produced a narrative on the attack. So did journalists who dug up additional facts on events leading up to the September 11 assault on America's financial and military brain center.
But the deaths of all 19 hijackers denied probers interrogations that likely would have produced even more information on the planning and on militant Islamist Osama bin Laden's global terrorist organization, al Qaeda.
Mr. Hamilton said another part of the plot remains a mystery. The hijackers devoted one plane, United Flight 93, to a target in Washington, D.C. But which one?
The answer is important because al Qaeda tends to revisit targets, such as the World Trade Center, on which a previous attack in 1993 failed.
"We know they want to attack symbols of American power," Mr. Hamilton said. "It would be interesting to know which symbols they put at the top, the top of their list."
The commission's report says bin Laden, who is still on the run from the U.S. military, wanted the fourth plane to hit the White House.
But Mohammed Atta, a key plot organizer and hijacker, thought the target too difficult and preferred the U.S. Capitol. Officials also speculated that CIA headquarters in McLean was the target.
Because of all this painstaking detective work, it is part of the historical record that Khalid Shaikh Mohammed, a Kuwaiti living in Pakistan, dreamed up the September 11 scheme and sold the plan to bin Laden.
It is known that bin Laden and his No. 2, Ayman al-Zawahri, personally recruited some of the 19 hijackers. Atta, who flew American Flight 11 into the trade center's North Tower at 8:46 a.m., was radicalized in a mosque in Hamburg, Germany.
Others found jihad at Islamic centers around the Middle East. All passed through bin Laden's fiefdom in Afghanistan before making their way to America, using passports and visas.
'Odd travel patterns'
A U.S. intelligence official, who asked not to be named, said the CIA still doesn't know with whom the hijackers met as they traveled across the United States.
Intelligence agencies would like to know "the nature and extent of support the hijackers had while in the United States, not to mention the nature and support in Germany and elsewhere," the official said in an interview. "We can't rule out as a possibility people who either knowingly or unwittingly" aided the conspirators.
The official asked, "What was behind their odd travel patterns? Why all the trips to Las Vegas? What exactly may have happened there? There are still some unanswered questions."
September 11 commissioner Fred Fielding, White House counsel to President Reagan, has his own pet mystery.
"The one thing that always bothered me, and we never got to the bottom of," Mr. Fielding said, was why Atta and fellow hijacker Abdulaziz Alomari drove from Boston to Maine to catch a flight back to Boston to connect with American Flight 11.
"What doesn't make sense to me is, this plot had to be perfect," Mr. Fielding said in an interview. "Everything had to work right for these guys, and unfortunately, it did. They couldn't have any mistakes.
"Atta and his team went up to Portland, Maine, and they flew back and had to go through two security checks. ... I've never figured it out. Why did he go to Portland, and what [did they do] in Portland, because it doesn't make any sense. He put the whole plan in jeopardy."
FBI Special Agent James N. Fitzgerald told the September 11 commission that the bureau has not figured out why Atta went to Maine.
"So the best indication we have of why he did what he did is from that detainee reporting, indicating that he probably did so to minimize the amount of people who would be arriving at Flight 11 at one time," Mr. Fitzgerald said.
Why it matters
Some of the unsolved mysteries -- such as with whom the hijackers met -- are important in understanding the plot's true breadth and whether any conspirators remain at large in the U.S., planning the next attack.
Other questions asked by the FBI, the commission and lawmakers would, if answered, shed more light on how al Qaeda terrorists operate in the days leading up to an attack. Such a profile might tip off the next attempt at mass murder.
Among other unanswered questions:
•More conspirators. Investigators believe there were other September 11 plotters. For instance, Atta took several trips to Europe after first arriving in the United States. The presumption is he met with jihadist allies -- but who? When Atta returned to Florida from one trip, fellow conspirator Marwan Al-Shehhi went to Casablanca, Morocco, for eight days. But why?
•Money. Investigators estimate the total cost of executing the attack at about $500,000, but do not know the ultimate source. Khalid Shaikh Mohammed, who is in CIA custody, handled most of the funds, but apparently cannot cast much light on its original source.
•The 20th hijacker. Three of the hijacked jets carried five jihadists; four hijackers were aboard the fourth, United Airlines Flight 93, which crashed in a field near Shanksville, Pa., after passengers attempted to retake the plane. The FBI's theory is that Mohammed planned for a 20th, but who?
The bureau initially said that Zacarias Moussaoui was to be that man. Moussaoui took flight lessons in Oklahoma and received $14,000 from conspirator Ramzi Binalshibh. But the government didn't present any evidence when Moussaoui pleaded guilty to conspiracy. He was sentenced to life in prison.
The September 11 commission said as many as eight other al Qaeda members attempted to enter the country but failed, including Mohammed al-Kahtani. The U.S. later captured al-Kahtani in Afghanistan and shipped him to a detention facility at the U.S. Naval Base Guantanamo Bay, Cuba.
Binalshibh, another captured al Qaeda member, was denied entry into the U.S. four times. Authorities eventually captured him in 2002.
Conspiracy theorists have had a field day with the September 11 attacks. Exploiting gaps in knowledge, fuzzy photographs and mistaken eyewitnesses, Internet bloggers circulated reams of unsubstantiated assertions.
Among these unlikely claims:
•The U.S. government planned and carried out the hijackings. How else to explain why none of the four planes was shot down by the military?
•The plane that hit the South Tower was not a Boeing 767 but a military refueling jet. Bombs must have caused the collapse of the twin towers because burning jet fuel is not hot enough to melt steel.
•A missile, not a Boeing 757, hit the Pentagon. French author Thierry Meyssan even knows what kind. He wrote in a book that it was a satellite-guided missile.
Now, a new book from Popular Mechanics magazine takes on the conspiratorialists. "Debunking the 9/11 Myths" examines 20 of the most persistent conspiracy theories and counters them with facts.
Reporters and editors say they consulted with more than 300 specialists and organizations to reach their conclusions. The book does not look at the al Qaeda plot, per se, but instead focuses on the major events that day, such as the fall of the twin towers.
For example: Maybe burning jet fuel is not hot enough to melt steel, but it did create an inferno hot enough -- when coupled with burning rugs, furniture and curtains -- to significantly weaken the frames, leading to the collapse.
About the Pentagon strike, "Debunking the 9/11 Myths" starts with the fact that dozens of persons actually saw American Flight 77 slam into the building's "E Ring" and bore toward the center courtyard. Then there is the plane's debris, scattered on the lawn.
"I held parts of uniforms from crew members in my hands, including body parts. OK?" structural engineer Allyn E. Kilsheimer told the book's authors.
"We are not attempting to tell the story of September 11," said James B. Meigs, Popular Mechanics editor in chief. "What we are doing is fact-checking the factual claims made by conspiratorial theorists themselves.
"They ignore the vast body of evidence. They ignore the huge accumulation of historical facts that confirms the mainstream view."
Critics of that view discount the entire story of United Flight 93, in which passengers talked on cell phones to authorities and loved ones before mounting an assault on the hijackers. The critics contend cell phones do not work in high-flying jet airliners.
But Mr. Meigs' reporters went to specialists who explained: "Cell phones work pretty well in airplanes, especially in rural areas."
Bloggers also wrote that World Trade Center building No. 7 must have been felled by demolition professionals because it received little damage in the attack. An early government report seemed to substantiate the damage angle.
Popular Mechanics discovered that report was wrong. The building's facade had been damaged significantly from falling debris, subsequent examinations showed.
And, skeptics ask, how could NORAD (North American Aerospace Defense Command) fail to launch intercepts to shoot down the four planes in the minutes after the Federal Aviation Administration determined they were hijacked?
NORAD, however, was ill-equipped to quickly respond and shoot down a civilian airliner. The command lacked streamlined communications with the FAA.
"The way the conspiracy theorists work is, they exploit the way they think things ought to work," Mr. Meigs said. "I call it the 'myth of government hyper-competence.'"
One glaring gap at the World Trade Center crime scene is the body count. Some victims simply vanished. The extensive fireball incinerated some to dust, making recovery impossible.
Of the 2,749 persons killed, the remains of 1,151 have not been identified.
Last spring, nearly 500 human remains were found on the roof of the Deutsche Bank building at ground zero by workers preparing the site for demolition, offering hope that the remains of more of those killed might yet be identified.
The grisly discovery prompted a coalition of September 11 survivors' organizations to ask that demolition of the building be halted until after a thorough search of the site.
"The huge amount of human remains recently found after nearly five years, and the prediction from experts that large amounts will continue to be discovered, is cause for great concern," the coalition announced. "Now is the time to take action to recover the over 40 percent of victims still missing."
Federal, state and local authorities made a massive effort to identify the remains of missing victims until New York officials suspended the process.
The last official identification took place in January: New York firefighter Keithroy M. Maynard, 30, of Engine Co. 33. His family had held off having a funeral until DNA testing identified his remains -- four years after the attack.
City and state officials have said they expect to find more remains as demolition and construction work near ground zero continues.
Despite the use of advanced DNA techniques, the remains of nearly 42 percent of those killed at the World Trade Center have not been found. Of the more than 19,900 separate remains recovered from ground zero, 4,735 were identified.
Only 293 intact bodies were found, authorities said, of which only 12 could be identified by sight. Nearly 10,000 remains have been frozen for future analysis.
Dr. Robert C. Shaler doubts that all those killed will be accounted for. As the director of forensic biology at the New York City Office of Chief Medical Examiner, he was responsible for overseeing DNA identification of the World Trade Center victims.
Some of the victims likely were incinerated amid the burning rubble, Dr. Shaler said in an interview with The Washington Times. Others were so decomposed that DNA testing may not be possible.
Dr. Shaler is now a professor of biochemistry and molecular biology at Penn State University, where he is director of the forensics science program.
In his book, "Who They Were," Dr. Shaler writes that New York City was unprepared for such mass fatalities. The Office of the Chief Medical Examiner, he says, had to be reconfigured to process and identify the dead.
Authorities say the September 11 plot will remain an open case for years. One key reason: Any new tidbit can help stop the next conspiracy.
"This has been and continues to be an area of focus for the CIA and other intelligence agencies, because the answers could help deter or disrupt future terrorists attacks," the intelligence official said.
"Some questions may be answered with the passage of more time. But others may not be, because the precise details might be known only by the hijackers themselves."
And Mr. Hamilton, the commission vice chairman, cautions that his panel's voluminous report and archive of documents are not the final word on how the attack was carried out.
"We understood that we were writing a first draft of history," he said. "We tried very hard to make it factually accurate.
"Commissioners asked again and again and again, 'What are the facts?' We didn't claim to be infallible here. This will be an event, like the Kennedy assassination, that will be examined by generations to come."
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