Here are 10 questions about 9/11 that remain unanswered.
1. Did the CIA cover up its advance knowledge of at least two of the 9/11 hijackers?
Richard Clarke, the national counterterrorism czar on 9/11, thinks so. In an interview for an upcoming radio documentary, Clarke claimed that top-level CIA officials deliberately withheld from the White House and the FBI knowledge as early as 2000 that two al Qaeda members - Nawaf al-Hazmi and Khalid al-Mihdhar - were living in San Diego.
The former anti-terror chief said he believes that the CIA kept the info under wraps because it wanted to recruit the two Saudis to serve as double agents within bin Laden's organization. Instead, the two terrorists ended up hijackers on American Flight 77. George Tenet, who was CIA director, claims that Clarke is "reckless and profoundly wrong."
2. How strong is the connection between the 9/11 site cleanup and cancer and other diseases?
Last week, the New York City Fire Department's Dr. David Prezant published research in the prestigious medical journal Lancet showing that male firefighters who responded to 9/11 now have a cancer rate that's 19 percent higher than unexposed co-workers. That comes on top of earlier reports of higher rates of asthma and post-traumatic stress disorder among the responders at Ground Zero.
Indeed, the real questions are a) Why was the Bush administration so lax in issuing warnings about the toxicity of the site in 2001? and b) Why did it take a comedian, Jon Stewart, to shame Congress into funding a health-care bill for the ailing heroes of Ground Zero?
3. Who was really in charge on the morning of 9/11 - Bush or Cheney?
The administration claimed that at some point shortly after the initial 9/11 attacks, President George W. Bush - speaking in Sarasota, Fla. - gave Cheney the verbal OK for an order to shoot down hijacked planes if necessary, which Cheney then passed down the chain of command.
But there's no record of such a call. In 2006, Newsweek reported that "none of the [9/11 Commission] staffers who worked on this aspect of the investigation believed Cheney's version of events" about the call - but bureaucratic wrangling kept that out of the final report.
Cheney recently insisted that the call took place, but he added more mystery when he admitted that he had urged Bush not to rush back to the White House. Was that for the president's safety, or did the man whom some aides called "Edgar" - after famed ventriloquist Edgar Bergen - have other motives?
4. Why did NORAD mislead investigators about what happened on 9/11?
In the days following the 2001 attacks, officials assured the public that the military did get planes in the air quickly and was ready to shoot down the final jet, United Flight 93, if it had come near D.C.
Investigators for the 9/11 Commission concluded that generals provided false information - claiming, for example, that they responded to the Flight 93 hijacking at 9:16 a.m. when tapes proved the jet wasn't even hijacked until 12 minutes later. The Washington Post reported in 2006 that commission staffers debated referring their suspicions to the Justice Department for a possible criminal probe.
"I was shocked at how different the truth was from the way it was described," said John Farmer, the top lawyer for the commission.
But why lie? Most likely it was to cover up incompetence, but the true reason is still a mystery.
5. Did top Saudi officials provide financial support for the hijackers?
In a new book, The Eleventh Day, Anthony Summers and Robbyn Swan make a compelling argument that the Saudi royal family paid what amounted to "protection money" to bin Laden as early as 1995 and that there may have been contacts between Saudi representatives in the United States and some of the hijackers in the months leading up to 9/11.
In July 2002, three Saudi princes met bizarre deaths within a week of each other - allegedly after they had been named in the interrogation of a captured al Qaeda member, Abu Zubaydah. Coincidence?
6. Who killed five Americans with anthrax in fall 2001?
The initial belief was that the attacks were linked to 9/11 either through al Qaeda or Iraq's Saddam Hussein, but forensics showed that the biological weapon came from American stockpiles. One U.S. researcher was publicly named, then exonerated. In 2008, the government announced that its newer prime suspect - a scientist at Maryland's Fort Detrick named Bruce Ivins - had committed suicide and that the case was considered closed.
But is it? Remarkably, a disputed U.S. Justice Department filing just this July claimed that Ivins didn't have access to the equipment needed to execute the attacks, causing some members of Congress to call for a new probe.
7. Did Pakistan's notorious spy agency, the ISI, support the 9/11 hijackers?
In the days after 9/11, there were numerous reports of links between the ISI - longtime supporters of Afghanistan's Taliban that had sheltered bin Laden before 9/11 - and the hijackers. For example, al Qaeda suspect Zubaydah, who fingered top Saudis, also named a high-ranking Pakistani air-force officer, Mushaf Ali Mir, who died in a plane crash in 2003 (sound familiar?).
Most of these questions are still not answered and probably won't be, given the sensitive relationship between the U.S. and nuclear-armed Pakistan. But it didn't clear things up when bin Laden was found hiding in Pakistan.
8. Why did so many Bush officials fixate on Iraq in the hours after the attacks?
Despite a lack of any evidence tying Saddam's Iraq to 9/11, Bush administration officials looked immediately toward Baghdad. Defense Secretary Donald Rumsfeld questioned whether to "hit S.H." - Saddam - "at the same time" while the Pentagon was still on fire, and Bush immediately pressed Clarke on whether there was an Iraqi connection.
Ten years later, U.S. troops are still in Iraq, but why? Was the motivation purely oil, or revenge for Saddam's assassination plot against Bush's dad, or an excuse to get American troops out of Saudi Arabia but keep them in the Persian Gulf?
9. What really happened aboard Flight 93?
In the immediate period after 9/11, based on accounts of phone calls from the hijacked passengers aboard the doomed flight, it was widely speculated that they'd succeeded in storming the cockpit, wrested control of the United jet and caused it to crash well before its intended target, reportedly the U.S. Capitol.
But transcripts from the recovered cockpit voice recorder offered evidence that although the passengers were indeed trying to enter the cockpit, there's nothing to suggest that they got there. Instead, comments from the hijackers led investigators to theorize that they crashed the plane on purpose, but the real story of the doomed jet will never be known.
10. Has the 9/11-fueled "war on terror" really made America safer?
The "pro" argument: Improved intelligence and domestic-security measures at airports and elsewhere, and regime change in Afghanistan, have led to no new major attacks inside the U.S., and bin Laden and many of his former lieutenants from a decimated al Qaeda are dead or behind bars.
The "con" argument: The $1 trillion-plus cost of the post-9/11 wars, including the completely unnecessary one in Iraq, the morally unconscionable torture program OK'ed by Bush and Cheney, and the gulag-like prison at Guantanamo have depleted not just America's coffers but its moral standing in the world, while inspiring a new generation of terrorists.
The answer? Get back to us on the 20th anniversary.