1. What did National Security Adviser Condoleezza Rice tell President Bush about al Qaeda threats against the United States in a still-secret briefing on Aug. 6, 2001?

Rice has suggested in vague terms that the president's brief - prepared daily by the CIA - included information that morning about Osama bin Laden's methods of operation - including hijacking. But when the congressional committee probing Sept. 11 asked to see the report, Bush claimed executive privilege and refused to release it.

2. Why did Attorney General John Ashcroft and some Pentagon officials cancel commercial-airline trips before Sept. 11?

On July 26, 2001 - 47 days before the Sept. 11 attacks - CBS News reported that Ashcroft was flying expensive charters rather than commercial flights because of a "threat assessment" by the FBI. CBS said, "Ashcroft has been advised to travel only by private jet for the remainder of his term." Newsweek later reported that on Sept. 10, 2001, "a group of top Pentagon officials suddenly canceled travel plans for the next morning, apparently because of security concerns."

Did either Ashcroft or the Pentagon have advance information about a 9/11-style attack and, if so, why wasn't this shared with the American public?

3. Who made a small fortune "shorting" airline and insurance stocks before Sept. 11?

On Sept. 10, 2001, the trading ratio on United Airlines was 25 times greater than normal at the Pacific Exchange, where traders could buy "puts," high-risk bets that the price of a company's stock will fall sharply. The next day, two hijacked United jetliners crashed, causing the company's shares to plummet and ultimately leading the airline into bankruptcy. CBS News later reported that at intelligence agencies, "alarm bells were sounding over unusual trading in the U.S. stock options market" on the day before the attacks.

The unusual stock trading suggests that someone with a sophisticated knowledge of finance also had advance information about the impending attack. But two years later, no one has been charged in this matter, and officials have not indicated even if the probe is still open.

4. Are all 19 people identified by the government as participants in the Sept. 11 attacks really the hijackers?

Probably not. Just 10 days after the attacks, a report by the British Broadcasting Corp. said that some of the supposed hijackers identified by the FBI appeared to be alive and well. The BBC story said Abdelaziz al-Omari, named as the pilot who crashed the jet into the World Trade Center's North Tower, was reported by Saudi authorities to be working as an electrical engineer. He reported his passport had been stolen in Denver in 1995. Saudi officials said it was possible that another three people whose names appear on the FBI list also are alive.

The article, which can be read at Unanswered Questions, makes a persuasive case that another man was posing as Ziad Jarrah, the alleged pilot of hijacked Flight 93, which crashed in Shanksville, Pa. So why did this story line vanish into thin air?

5. Did any of the hijackers smuggle guns on board as reported in calls from both Flight 11 and Flight 93?

Quite possibly. An internal Federal Aviation Administration memo written at 5:30 p.m. on the day of the attacks said that a passenger aboard American Airlines Flight 11 - Israeli-American Daniel Lewin - had been shot to death by a single bullet before the jet slammed into the North Tower of the World Trade Center. The FAA insists the memo was a mistaken "first draft," even though the alleged shooting is described in great detail.

Aboard Flight 93, passenger Thomas Burnett told his wife, Deena, in a 9:27 a.m. cell-phone call: "The hijackers have already knifed a guy, one of them has a gun, and they are telling us there is a bomb on board."

Why has this angle of Sept. 11 not been investigated in more detail?

6. Why did the NORAD air defense network fail to intercept the four hijacked jets?

During the depths of the Cold War, Americans went to bed with the somewhat reassuring belief that jet fighters would intercept anyone launching a first strike against the United States. That myth was shattered on 9/11, when four hijacked-jetliners-turned-into-deadly-missiles cruised the American skies with impunity for nearly two hours.

Why did the North American Aerospace Defense Command seem unaware of literally dozens of warnings that hijacked jetliners could be used as weapons? Why does NORAD claim it did not learn that Flight 11 - the first jet to strike the World Trade Center about 8:45 a.m. - had been hijacked until 8:40 a.m., some 25 minutes after the transponder was shut off and an astounding 15 minutes after flight controllers heard a hijacker say, "We have some planes..."?

Why didn't the fighters that were finally scrambled at Otis Air Force Base in Massachusetts and Langley Air Force Base in Virginia fly at top, supersonic speeds? Why didn't fighters immediately take off from Andrews Air Force Base, just outside Washington, D.C.? Why was nothing done to intercept American Airlines Flight 77, which struck the Pentagon, when officials knew it had been had been hijacked some 47 minutes earlier?

And why has no one been disciplined for the worst breakdown in national defense since Pearl Harbor?

7. Why did President Bush continue reading a story to Florida grade-schoolers for nearly a half-hour during the worst attack on America in its history?

In arguably the greatest understatement in U.S. history, Bush told a questioner at a California town-hall meeting in January 2002 that 9/11 "was an interesting day." Interesting, indeed. In the two years since the attacks, questions have only grown about the president's bizarre behavior that morning, when he was informed in a Sarasota classroom that America was under attack.

"I couldn't stop watching the president sitting there, listening to second-graders, while my husband was burning in a building," World Trade Center widow Lorie van Auken, a leader of relatives of Sept. 11 victims who have raised questions about the attacks, told Gail Sheehy in the New York Observer.

Why did Bush read a children's story about a pet goat and stay in the classroom for more than a half-hour after the first plane struck the World Trade Center and roughly 15 minutes after Chief of Staff Andrew Card told him that it had been a deliberate attack? Why didn't he take more decisive action, and why wasn't he hustled to a secure area while the attacks were clearly still under way?

Conspiracy advocates have cited these strange lapses as evidence that Bush knew about the attacks ahead of time, but why would anyone with advance knowledge appear so clueless?

For a fascinating read on the subject, go to: An Interesting Day.

8. How did Flight 93 crash in western Pennsylvania?

The most popular version - that heroic passengers who fought with the hijackers successfully stormed the cockpit - has become so widely accepted that people were jarred last month when an Associated Press report seemed to contradict it. The AP story took one line out of a congressional report and wrote that the FBI now believes the hijackers crashed the plane on purpose.

Many were dismayed that the FBI would change its story, but the government had never put out an official story. Some unidentified government officials had first floated the hijackers-crashed-the-plane-on-purpose theory in late 2001.

Based solely on circumstantial evidence from several cell-phone calls made by passengers, most of the public and the mainstream media have come to believe that the plane crashed because of a struggle between the passengers and the hijackers.

Meanwhile, the FBI reportedly has enough hard information about what really happened on Flight 93 to have worked up a flight-simulation video. But that video, the cockpit audio recording and the hard data from the other "black box," the flight data recorder, is still top secret.

The issue symbolizes the government's continuing refusal to release information about what really happened on Sept. 11. Even some relatives of Flight 93 victims are growing unhappy that more information has not been publicized.

9. Was Zacarias Moussaoui really "the 20th hijacker"?

Almost certainly not, even though the allegation has been repeated hundreds of times in the media. The Moroccan native, who has been in custody since his August 2001 arrest on immigration charges after he attended a flight-training school in Minneapolis, has admitted that he is a member of al Qaeda and wanted to commit terrorist acts in America. But he arrived here much later than the Sept. 11 hijackers and reportedly had no contacts with them.

The issue is important because some family members of Sept. 11 victims who are seeking information about what happened that day have been turned down because of the ongoing Moussaoui case.

10. Where are the planes' "black boxes"?

Nothing is more critical to learning about air disasters than the so-called "black boxes." They are the 30-minute audio recordings of cockpit chatter and the fight-data inputs which show the speed, direction and operational condition of the plane, and which are encased in material designed to withstand a high-speed crash. Yet the government has continued to keep a lid of secrecy on the black boxes from Flight 77, which crashed into the Pentagon, and from Flight 93.

FBI Director Robert Mueller has said Flight 77's data recorder provided altitude, speed, headings and other information, but the voice recorder contained nothing useful. Why not? Why not release the information to the public? Why has a docile mainstream media not demanded this information?

And how come none of the four "indestructible" black boxes was recovered from the World Trade Center, even as investigators said that a passport belonging to one of the hijackers had been found in the rubble, undamaged, a week after the towers's collapse?

11. Why were Donald Rumsfeld and other U.S. officials so quick to link Saddam Hussein to the attacks?

CBS News reported that the defense secretary was making notes about invading Iraq even before the fires from Flight 77 had been extinguished on the other side of the Pentagon. Rumsfeld wrote that he wanted "best info fast. Judge whether good enough [to] hit S.H." - Saddam Hussein - "at the same time. Not only UBL" - Osama bin Laden. He added: "Go massive. Sweep it all up. Things related and not."

Rumsfeld and a number of other Bush administration officials have ties to a once-obscure policy group called the Project for a New American Century. In a 2000 white paper, PNAC - which had long urged an American invasion of Iraq - said that for the United States to assert itself properly as the world's lone superpower, "some catastrophic and catalyzing event - like a new Pearl Harbor" - would be required.

That new Pearl Harbor came - two years ago today.

12.Why did 7 World Trade Center collapse?

7 World Trade Center, a 47-story building, was not struck by an aircraft on Sept. 11, yet the building mysteriously collapsed at 5:20 p.m. that afternoon. Apparently debris from the jetliner attacks on the adjacent twin towers started a fire at No. 7. But as the New York Times noted: "No building like it, a modern, steel-reinforced high-rise, had ever collapsed because of an uncontrolled fire." Investigators have speculated that excess diesel fuel for emergency generators fanned the flames, but the full story may never be known.

Some questions also have lingered about why the two 110-story towers collapsed. But investigators think the burning jet fuel - compounded by paper-and-electronics-laden cubicles and possibly insulation matter - burned long enough, at temperatures exceeding 1,000 degrees, to weaken the structural steel.

13. Why did the Bush administration lie about dangerously high levels of toxins and hazardous particles after the WTC collapse?

Because apparently some White House officials felt that the health of the American economy and Wall Street was more important than the health of New York City residents who lived nearby. For example, on Sept. 16, 2001, a draft press release from the Environmental Protection Agency said: "Recent samples of dust gathered by OSHA on Water Street showed higher levels of asbestos in EPA tests." That was deleted and replaced with this: "The new samples confirm previous reports that ambient air quality meets OSHA standards and consequently is not a cause for public concern."

A key figure in the changes was the head of the White House Council on Environmental Quality, who - you can't make this stuff up - is a lawyer who formerly represented the asbestos industry.

In fact, the EPA told workers and residents that it was safe to return to lower Manhattan at a time when some test results had not been analyzed and other key tests had not even been performed. The outcome? Key medical professionals say thousands of New Yorkers have developed respiratory illnesses associated with exposure to the dust. Symptoms include periodic gasping for air, a choking sensation and unusual sensitivity to airborne irritants, apparently from a type of "occupational asthma" called Reactive Airways Disease Syndrome.

14. Where is Dick Cheney's undisclosed location?

We'll never know, but a widely reported rumor was that it was right here in the Keystone State. The speculation is the vice president spent the days after the attack at Site R, a secretive Cold War-era site, also known as Alternate Joint Communications Center, deep inside Raven Rock Mountain. The mountain is in western Pennsylvania, near Waynesboro.

15. What happened to the more than $1 billion that Americans donated after the attack?

The largest recipient, the American Red Cross, says it already has used $741 million from its Liberty Fund to help more than 55,000 families cope with the death of loved ones, serious injuries, physical and mental health concerns, financial loss, homelessness and other effects of the attacks.

Of that, $596 million was in the form of direct financial assistance to families of those killed or seriously injured, as well as to displaced workers, residents and emergency personnel who were seriously affected. Depending on individual needs, this financial assistance included up to a full year's living expenses, estate and special-circumstances cash grants, and more.

16. What was the role of Pakistan's spy agency in the Sept. 11 attacks and the subsequent murder of U.S. journalist Daniel Pearl?

The idea that Pakistan is considered a leading American ally in the war on terror is both ironic and a bit disturbing when one considers that there are proven links between Pakistan's intelligence agency, the notorious ISI, and the Taliban, as well as likely ties to al Qaeda and bin Laden.

In October 2001, the Wall Street Journal and many reputable news organizations in South Asia reported that the head of the ISI, Lt. Gen. Mahmoud Ahmad, was fired after being linked to a $100,000 payment that had been wired to al Qaeda hijacker Mohamed Atta in America to pay for the Sept. 11 attacks. The New York Times said the intelligence service even used al Qaeda camps in Afghanistan to train covert operatives for use in a war of terror against India.

In recent weeks, two troubling reports have emerged. The highly regarded French journalist Bernard-Henri Levy has written that Wall Street Journal reporter Pearl had been murdered by elements of the ISI because he'd learned that al Qaeda "is largely controlled by the Pakistani secret service" and that Islamic extremists control the nation's nuclear weapons. And investigative reporter Gerald Posner writes that bin Laden lieutenant Abu Zubaydah not only revealed a link to top Saudis but also to high-ranking Pakistani air force officer Mushaf Ali Mir. Mir, who is said to have cut protection deals in secret meetings with bin Laden, died earlier this year in a plane crash that also killed his wife and closest confidants.

17. Who killed five Americans with anthrax?

Actually, it's not clear whether this question should even be on this list. Two years later, it's not known whether the anthrax-laden letters that killed five Americans from Connecticut to Florida, and targeted some leading Democratic pols and TV news anchors, had anything to do with the Sept. 11 attacks. Indeed, the list of potential suspects - al Qaeda terrorists, Saddam, crackpot U.S. scientists - hasn't been narrowed down. Our government's utter cluelessness about a reign of terror that rattled the nation and dominated the headlines in fall 2001 is an investigative failure of epic proportions.

One man, a former Army biomedical researcher named Steven J. Hatfill, has been labeled "a person of interest" by the FBI, but nothing definitive has linked Hatfill to the crime. Just this summer, federal investigators drained a Frederick, Md., pond where they speculated the anthrax letters might have been assembled, but tests of soil samples taken after the draining yielded no evidence of biological weapons. And now Hatfill has sued the government for invading his privacy - in a case that may never be solved.

18. What happened to the probe into C-4 explosives found in a Philadelphia bus terminal in fall 2001?

Do you remember this front-page headline from Oct. 20, 2001: "In Phila. locker, a lethal find; Explosive 'would probably have leveled' bus depot." You can be forgiven if you don't. There's been no mention in local media since late 2001 of the alarming discovery of one-third of a pound of lethal C-4 and 1,000 feet of military detonation cord in a locker at the Greyhound bus terminal in Center City, even though it's possibly the most direct link between Philadelphia and domestic terrorism.

Investigators conceded a couple of months into their probe that the trail had gone stone-cold. They speculated that the material had been stolen from an Army base and that the culprit, who rented the locker on Sept. 29, 2001, decided that the material was too hot to handle after the Sept. 11 attacks. The truth may never be known.

19. What is in the 28 blacked-out pages of the congressional Sept. 11 report?

It's not a total mystery. Everyone has acknowledged that the pages contain highly embarrassing information about links between the Sept. 11 hijackers and the government of Saudi Arabia, America's supposed ally in the Middle East and home to the world's largest oil reserves. One of those officials is said to be Saudi ambassador Prince Bandar, whose wife, Princess Haifa, indirectly funded at least two of the Sept. 11 terrorists during their time in San Diego. The prince is so close to the Bush family that he's known, incredibly, as "Bandar Bush." This week, Time reports that just after the Sept. 11 attacks, when U.S. commercial airspace was still closed to our citizens, Bush allowed a jet to stop at 10 U.S. cities to pick up and fly home 140 prominent Saudis, including relatives of bin Laden.

A new must-read book by investigative reporter Posner - "Why America Slept" - takes the conspiracy to the highest of levels of the Saudi government. He says a top bin Laden lieutenant, Abu Zubaydah, who was captured in March 2002, stunned investigators when - allegedly given the "truth serum" sodium pentothal - fingered three top Saudis. They were Prince Ahmed bin Salman bin Abdul Aziz, the Westernized owner of 2002 Kentucky Derby winner War Emblem; Prince Turki al-Faisal bin Abdul Aziz, the kingdom's longtime intelligence chief, and Prince Fahd bin Turki bin Saud al-Kabir.

The most incredible part of the story is what happened next. In an eight-day period in late July 2002, Prince Ahmed died at age 43 from a heart attack, Prince Turki died in a car crash and Prince Fahd "died of thirst." Coincidence? What do you think?

20. Where is Osama bin Laden?

Remember how President Bush vowed on Sept. 17, 2001, that he was determined to catch bin Laden "dead or alive"? Well, the good news is that if he wants bin Laden "alive," there's still a chance that could happen. Intelligence experts now agree that bin Laden successfully escaped his Tora Bora hideout in Afghanistan back in December 2001 - when the U.S. failed to commit ample manpower to the chase - and that the al Qaeda leader is alive and well, and plotting new attacks.

"We don't know where he is," Army Col. Rodney Davis, spokesman for America's forces in Afghanistan, said recently. But Newsweek seems to know where to find bin Laden: in the remote, mountainous - and lawless - Kunar province of Afghanistan. The magazine chillingly reported that just five short months ago, bin Laden convened the biggest terror summit since Sept. 11 at a mountain stronghold there. The participants reportedly included three top-ranking representatives from the Taliban, several senior al Qaeda operatives and leaders from radical Islamic groups in Chechnya and Uzbekistan. The topic was carrying out attacks against U.S. interests inside Iraq.

The most chilling aspect of the Newsweek report is that bin Laden has access to biological weapons and is determined to find a way to use them against the United States. A source from the Taliban told the magazine: "Osama's next step will be unbelievable."

But this week, ABC News reported that the hunt for bin Laden has been narrowed to a different area - a 40-square-mile section of the Waziristan region of Pakistan. The report said that local residents suspected of trying to inform Americans about bin Laden's whereabouts were executed in broad daylight.