It is a question at the forefront for many relatives of Sept. 11 victims, who have been frustrated and confused by answers from aviation and defense officials about an emergency response they believe was woefully inadequate.
"When you think about a criminal investigation, your first step is a timeline of events. But nearly three years after the attacks, we still don't know what happened," said Mindy Kleinberg, whose husband, Alan, was killed in the World Trade Center collapse.
Commission members say the hearing will provide some definitive answers, with a staff report faulting, in part, the chaos and poor communication between the Federal Aviation Administration and the North American Aerospace Defense Command.
Another critical failing, panel members have said, was NORAD's post-Cold War focus on intercepting Soviet bombers rather than hijacked airliners. Aviation and military officials have acknowledged problems during the attacks, but say they've made improvements since then.
"The real issue is first establishing the facts minute-by-minute," said Republican commissioner John Lehman, former secretary of the U.S. Navy. "Who knew what when? What orders were given? From there we can learn the lessons of what went right.
"For the first time, you will see a clear picture of what happened that everyone can understand and not dispute," he said.
Democratic commissioner Jamie Gorelick, a former deputy attorney general in the Clinton administration, said the hearing will also highlight some success stories that day.
"You'll see some heroic efforts made. There were some very talented people operating at peak capacity and improvising when faced with a challenge they had not planned for," she said.
In the hours just after the attacks, nearly 4,500 planes had to be landed as quickly as possible. While stunned colleagues were glued to TVs nearby, air traffic controllers first had to focus on rerouting about a quarter of them — 50 times as many planes as they usually reroute each hour.
The responses by President Bush and Vice President Dick Cheney also will be reviewed, including when a shootdown order was delivered to NORAD.
Scheduled to testify Thursday are Air Force Gen. Richard Myers, chairman of the Joint Chiefs of Staff, as well as officials from NORAD and the FAA.
The commission, facing a July 26 deadline for a final report, is winding down its 1½-year investigation after interviewing more than 1,000 witnesses, including Mr. Bush, and reviewing more than 2 million documents.
On Wednesday, the commission opened a two-day hearing into the Sept. 11 plot. It found the terror network had originally envisioned a much larger attack and was working hard to strike again, most likely in the form of a chemical, radiological or biological attack.
The commission staff said Sept. 11 mastermind Khalid Shaikh Mohammed initially outlined an attack involving 10 aircraft targeting both U.S. coasts. Mohammed proposed that he pilot one of the planes, kill all the male passengers, land the plane at a U.S. airport and make a "speech denouncing U.S. policies in the Middle East before releasing all the women and children," the report said.
Bin Laden rejected that plan as too complex, deciding instead on aircraft piloted by hand-picked suicide operatives. The report said the targets were chosen based on symbolism: the Pentagon, which represented the U.S. military; the World Trade Center, a symbol of American economic strength; the Capitol, the perceived source of U.S. support for Israel; and the White House. Training for the attacks began in 1999.
The attacks were planned for as early as May 2001, but they were pushed back to September, partly because al Qaeda sought to strike when Congress would be at the Capitol. A second wave of hijackings never materialized because Mohammed was too busy planning the Sept. 11 attacks, the report said.
Commission member Bob Kerrey, a former Democratic senator, said, "I believe that we missed a tremendous opportunity very early in this game to inform the Congress, inform the American people who bin Laden was, what he was doing, what he had done, and as a consequence I think we simply didn't rally until it was too late."
The staff report also concluded no evidence exists that al Qaeda had strong ties to Saddam Hussein.
Although Osama bin Laden asked for help from Iraq in the mid-1990s, Saddam's government never responded, according to the report. The report asserted "no credible evidence" has emerged that Iraq was involved in the Sept. 11 strikes.
In making the case for war in Iraq, Bush administration officials frequently cited what they said were Saddam's decade-long contacts with al Qaeda operatives.
In recent days, administration officials have reiterated their claims of a link. Cheney said Monday at a reception in Florida that Saddam "had long established ties with al Qaeda."
Secretary of State Colin Powell, asked about the commission report, said the administration stands by its assertions that there were links between al Qaeda and Iraq.
"I think we have said, and it is clear, that there is a connection, and we have seen these connections between al Qaeda and the regime of Saddam Hussein and we stick with that," Powell said in an interview on the al-Jazeera television network. "We have not said it was related to 9/11."
The White House also released an Oct. 7, 2002, letter from the CIA to then-Senate Intelligence Committee Chairman Bob Graham citing "solid reporting" of an Iraq-al Qaeda relationship "going back a decade" and credible reports that al Qaeda sought contacts in Iraq who might help them acquire the capability to use weapons of mass destruction.
A spokesman for British Prime Minister Tony Blair said that while there was no direct link, Saddam "created a permissive environment for terrorism and we know that the people affiliated to al Qaeda operated in Iraq during the regime."