Almost 2 inches thick and 850 pages long, the congressional inquiry into the Sept. 11 attacks largely boils down to two intriguing words: What if?
What if an FBI agent had known that two acquaintances of his San Diego informant had been identified as terrorists and would turn out to be among the hijackers? What if agencies had shared more information? What if intelligence agencies had more spies in Afghanistan? What if fighting Usama bin Laden (search) had been a higher priority?
The final report Thursday of the House and Senate intelligence committees' inquiry into the attacks identifies countless blunders, oversights and miscalculations that prevented authorities from stopping the attacks.
"Significant pieces of information in the vast stream of data being collected were overlooked, some were not recognized as potentially significant at the time and therefore not disseminated, and some required additional action on the part of foreign governments before a direct connection to the hijackers could have been established," the report said. "For all those reasons, the intelligence community failed to fully capitalize on available, and potentially important, information."
But even if some or most of those problems hadn't occurred, would that have been enough to save 3,000 lives?
Sen. Bob Graham, the inquiry's co-chairman, believes so.
"The attacks of Sept. 11 could have been prevented if the right combination of skill, cooperation, creativity and some good luck had been brought to task," said the Florida Democrat, who also is running for the presidency.
But the other co-chairman, Rep. Porter Goss, R-Fla., seemed less certain. Much of the Sept. 11 plot remains a mystery.
"I can tell you right now that I don't know exactly how the plot was hatched on 9-11," he said.
Graham and Goss still know a lot more than the public does. The report released Thursday followed a battle between congressional staff and intelligence agencies over what could be released from the full classified report completed in December.
Many details in the report remain secret, including a 28-page section on foreign support for the hijackers that was almost entirely redacted. Questions have been raised about whether hijackers have received direct or indirect funding from Saudi Arabia (search).
Graham and two other inquiry leaders, House Democratic Leader Nancy Pelosi of California and Sen. Richard Shelby, R-Ala., said too much remains classified.
"Classification certainly must protect sources and methods, but it should not be used to protect reputations," Pelosi said.
But Goss said the classification was needed to keep terrorists from learning about counterterrorism operations. "I think we've got the balance about right, right now," he said.
An independent commission headed by former New Jersey Gov. Thomas Kean (search) is currently following up on the joint inquiry's report, taking a broader look at the Sept. 11 attacks. The commission's purview includes non-intelligence issues such as aviation security and immigration. It is expected to issue its findings by May.
Despite the censored information, the report released Thursday contained a wealth of new details about missed opportunities to unravel the plot.
It included what the report described as the "intelligence community's best chance to unravel the Sept. 11 plot" - an FBI informant in San Diego who knew two of the future hijackers. The San Diego FBI office wasn't aware the two men already had been linked to Al Qaeda.
The report also chastises the CIA for giving little credence to intelligence gathered in spring 2001 that said terrorist Khalid Shaikh Mohammed (search) was seeking recruits to travel to the United States. Mohammed was later identified as a mastermind of the attacks.
It also said the military was reluctant to launch attacks on bin Laden in Afghanistan, partly because intelligence on his whereabouts was not specific or of questionable reliability.
The report cites a number of warnings of attacks by Al Qaeda (search) in the United States and against aviation. One, in December 1998, said "plans to hijack U.S. aircraft proceeding well. Two individuals ... had successfully evaded checkpoints in a dry run at a N.Y. airport."
President Bush, responding to the report, said in a statement: "Our law enforcement and intelligence agencies are working together more closely than ever and are using new tools to intercept, disrupt and prevent terrorist attacks."
CIA and FBI officials say they have already addressed many of the deficiencies, particularly in targeting Al Qaeda and communicating with one another.
The problems of agencies sharing information within their organization and with each other has long been seen as one of the main intelligence failures. Separate warnings from Phoenix and Minneapolis FBI offices that terrorist groups might be training pilots were never linked.
Lawmakers often talk about a failure to "connect the dots." That term has annoyed some intelligence officials, suggesting that linking bits of information from a vast number of intelligence leads was as simple as a child's game.
At an inquiry public hearing in September, longtime FBI counterterrorism official, Dale Watson, said clues inevitably become much more obvious after the fact and suggested a maze is a better analogy.
"If you know where the end point of a maze is, it's certainly easier to work your way back to the starting point," he said.