One of the central chapters in the Sept. 11 report released last week is a painstaking reconstruction of the hijackers' preparations.
It covers their arrival in the United States, their training at flight schools, their purchases of tickets and their boarding of the doomed aircraft.
But Page 139 points to a lingering mystery: During cross-country practice flights in the preceding months, why did the hijackers always book return stops in Las Vegas?
"Each of the return flights for these hijackers had layovers in Las Vegas," FBI Director Robert S. Mueller III is quoted as saying in the report. "To date, the purpose of these one- to- two-day layovers is not known."
Indeed, for all that it answers about the attacks, the nearly 900-page report is stocked with reminders of the many questions that remain -- about other puzzling aspects of the plot, the possible role of foreign governments and even such politically charged matters as what Presidents Clinton and Bush had been told about Al Qaeda.
In that sense, the release of the congressional report last week marks only the latest step in the nation's struggle to understand the attacks and the government's performance leading up to them.
The next phase is well underway. An independent Sept. 11 commission, led by former New Jersey Gov. Thomas Kean, is taking an even broader and deeper look, probing not just intelligence failures but breakdowns in other areas of government, including immigration and aviation.
Already, commission officials say they have been granted access to a trove of materials congressional investigators never got to see, including reports on interrogations of top Al Qaeda figures in custody, as well as records from the National Security Council inside the White House.
And officials at the commission, which is expected to deliver its report next year, say they are still pressing for more.
"We are requesting access to things to which no prior White House has granted access, ever," said Philip Zelikow, executive director of the commission. He declined to discuss details of those negotiations with the administration, but said: "The bottom line is: 'So far, so good.' "
Meanwhile, even lawmakers privy to the fuller, classified version of the report of the joint congressional inquiry acknowledge that the picture is incomplete.
"I can tell you right now that I don't know exactly how the plot was hatched," Rep. Porter J. Goss (D-Fla.), co-chairman of the joint inquiry, said last week.
"I don't know the where, the when and the why and the who in every instance. That's after two years of trying. And we will someday have the documents to exploit, we will have the people to interrogate, we will have ways to get more information to put the rest of the pieces of this puzzle on the table. But right now, we don't have it."
One big reason there are holes in the story is that until recently, U.S. officials have had little access to persons involved in planning the attacks. That makes the interrogation transcripts particularly intriguing for the new commission.
Among the Al Qaeda suspects now in U.S. custody are Khalid Shaikh Mohammed, the alleged mastermind of Sept. 11, and Abu Zubeida, a top lieutenant to Osama bin Laden. If they are talking -- and telling the truth -- the commission could piece together the most definitive account yet of the origin and execution of the plot.
Although it is incomplete, the joint inquiry report provides some new insight on this subject. One passage cites testimony from CIA Director George J. Tenet that in 1996, Bin Laden's second-in-command, Mohammed Atef, drew up a study on the feasibility of hijacking U.S. planes and destroying them in flight. Mohammed proposed to Bin Laden that the World Trade Center "be targeted by small aircraft packed with explosives." Bin Laden then suggested using even larger planes.
The sources Tenet relied on in piecing together this account are unspecified.
Although the congressional report contains gaps, it does provide a comprehensive look at the performance of the CIA, the FBI and other agencies in the intelligence community leading up to the attacks.
The overriding finding was that there were repeated opportunities to anticipate or disrupt the Sept. 11 plot, from unheeded FBI memos warning that Al Qaeda was sending terrorists to U.S. flight schools to CIA failures to put Al Qaeda operatives on U.S. watch lists.
The report is full of tantalizing glimpses into covert activities around the globe in the late 1990s, as U.S. intelligence tried desperately to get close enough to capture or kill Bin Laden. But many of the most intriguing passages end abruptly, with the most sensitive information blotted out at the insistence of the White House or CIA.
In one section, Samuel R. "Sandy" Berger, former national security advisor to President Clinton, said that in early 2000 the National Security Council, the military and the CIA had developed a "new technique for detecting Bin Laden." Berger describes it as "very promising as a way of determining where he would be if we had one strand of human intelligence."
But whether the new technique was some sort of high-tech homing device or silent surveillance craft is a question left to future generations. The subsequent 2 1/2 pages are blank.
And besides, Berger and others make clear elsewhere in the report that getting even a "strand of human intelligence" on Bin Laden was typically more than U.S. intelligence could manage.
Other deleted portions of the text point to more urgent questions. The largest gap in the report is a 27-page section dealing with information "suggesting specific sources of foreign support for some of the Sept. 11 hijackers while they were in the United States."
Although the report does not name any countries, other sources said much of the omitted material dealt with information suggesting hijackers got significant support from Saudi officials, an explosive claim the Saudi government vehemently denies.
Lawmakers vowed to continue to press for the release of such material.
"Those pages will not be blank forever," Goss said.
But how long they will remain blank is anybody's guess. A congressional official involved in producing the report said the material could be declassified "in two years or 50 years. But I'd bet on the 50 years."
In other areas, congressional investigators encountered predictable road blocks. The White House is loath to expose its inner workings to Congress, and generally isn't obligated to do so because of the constitutional principle that the executive and legislative branches are coequal.
Politics also plays a role. The congressional inquiry was co-chaired by a Democratic senator, Bob Graham of Florida, who is now running for president. The Bush administration may consider it less politically risky to share certain information with the independent commission, which is seen as less partisan.
But the commission has encountered obstacles of its own, and it could face more clashes as it presses for information from the president's inner circle.
Former Indiana Rep. Tim Roemer, a Democrat who served on the congressional probe and was appointed to the independent commission after he left Congress last year, said many of the most pressing questions are also among the most uncomfortable for the White House.
"We need to look and see whether the Clinton and Bush administrations were aggressively pursuing Al Qaeda," Roemer said. "Were they slow to prioritize it, identify it and get agencies to act on it?"
Among the key tasks for the commission, Roemer said, is to get direct testimony from such officials as national security advisor Condoleezza Rice. Other commissioners have said they have requested private diaries and notes from current and former administration officials.
Complicating the White House's consideration of such issues is the fact that Congress has launched separate inquiries into the prewar intelligence on Iraq. The administration may find it hard to cooperate with one inquiry and not another.
Meanwhile, the FBI and CIA inspectors general are conducting their own "accountability" studies of their agencies' performance leading up to Sept. 11. Some of their findings could be shared with the commission. Others might come to light only if they are declassified decades from now.
Then there are the Las Vegas layovers. Did the devout Muslim hijackers simply partake of the gambling mecca's pleasures, or was there a more strategic explanation? Another puzzler: Why did ringleader Mohamed Atta and another hijacker spend the night before the attacks in Portland, Maine, which forced them to rely on a shuttle flight to Boston to catch the plane they hijacked?
"I still don't think we know about the 19 hijackers -- where they were, why they did certain things," Roemer said. Explaining the attacks, he said, "is like the myth of Sisyphus; you're continually pushing this rock up the mountain."