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How were the hijackers armed?

During the trial of the suspected terrorist Zacarias Moussaoui, prosecutors submitted exhibit GX-FO08301, a box cutter: yellow plastic, with a black curved handle, made in China. It had been found in a rental car left by the terrorists at the airport in Washington. A superpower brought to its knees by assailants with box cutters - at least that's the story.

In fact, most of the hijackers were armed with regular knives. A passenger on flight AA 77, which smashed into the Pentagon, made the only reference to "knives and box cutters" during a phone call. Cell phone users from other planes said only that someone had been stabbed, but they didn't identify the weapon. Credit card records reveal that the hijackers bought everyday knives in at least two cases.

Inside the cabins, the terrorists also used pepper spray or other noxious chemicals. On three of the four flights, the hijackers claimed they had bombs on board.

Where and how were the combatants among the hijackers trained?

Abu Turab al-Urduni, a Jordanian al Qaeda veteran, was assigned this mission. Ten of the hijackers were trained at the al-Matar and al-Faruk camps in Afghanistan at the turn of 2000/2001, according to the director of al Qaeda operations, Chalid Sheikh Mohammed. The training included English, bodybuilding and techniques to disarm aircraft or security personnel. All ten had to slit the throat of a sheep and a camel using a Swiss Army knife.

What was the greatest risk for the operation?

According to Chalid Sheikh Mohammed, it was bin Laden himself. Months before the attacks, bin Laden apparently failed to maintain any operational security. In the summer of 2001, he repeatedly predicted a major attack on the United States. Mohammed and an associate urged bin Laden to show more discretion.

Had September 11 always been the date for the planned attacks?

No. Osama bin Laden actually wanted to strike earlier. May 12, 2001 - exactly seven months after the attack on the USS Cole - was one proposed date. Then, he got wind of a visit by Ariel Sharon, Israel's former prime minister, to the White House that summer. Sheikh Mohammed said he was able to talk him out of it both times, explaining that the teams weren't ready yet. Bin Laden reportedly became so impatient that he suggested simply crashing the hijacked planes rather than flying them into buildings.

Were the hijackers all fanatics?

Not to judge by their lifestyles. While Mohammed Atta was particularly devout and abstemious, another of the pilots - Ziad Jarrah - enjoyed partying and the occasional beer. Much to the chagrin of his accomplices, he even had a German-born Turkish girlfriend: Aysel S. This relationship troubled al Qaeda leaders until the very end. As a result of this relationship, they considered Jarrah the least dependable member of the group. Jarrah may have considered abandoning the plot and returning to Aysel. But just before the attacks, he chose death over love. On the morning of the flight, he called his girlfriend in Germany to bid farewell and told her he loved her three times over.

How was CNN able to provide live coverage so soon after the first plane had hit the North Tower?

It took three minutes for the world to be told of the catastrophe unfolding on September 11. Flight AA 11 rammed the North Tower at 8:46 a.m.; CNN broadcast the first live shots of the burning building as "breaking news" at 8:49 a.m. with the headline "Plane crashes into World Trade Center tower." The footage was shown simultaneously on CNN's 800 international affiliates.

The instant live coverage is easily explained: CNN had installed fixed cameras at various New York landmarks to provide scenery for live stand-up spots. The World Trade Center was a favorite interview backdrop for CNN Financial News, a nowdefunct CNN subsidiary. The camera that captured the burning North Tower was located two miles away on the roof of a skyscraper.

Why did the towers fall so quickly?

The engineers and fire-prevention experts agree on one thing at least: Both towers would have remained standing much longer had the fireproof coating on their frames not been destroyed in the crashes. Whether they would have survived completely remains a matter of debate.

A May 2002 study by the Federal Emergency Management Agency (FEMA) concluded that the towers had been structurally sound, and that the builders could not be held liable for their collapse. In 1973, when the buildings were completed, they were designed to withstand the impact of a Boeing 707. But 28 years later, the two Boeing 767s that hit the towers were traveling at 475 and 590 miles per hour, respectively, generating considerably more kinetic energy than a lone 707.

A 2005 report by the National Institute of Standards and Technology (NIST) blamed the damaged fireproofing for the catastrophe. Fires weakened the steel frame that supported the weight of the building. This caused several sections of flooring to sag. Unable to withstand the weight of the floors, the steel structures first buckled inward and then caved in completely.

Skeptics doubt that the blaze was hot enough to melt steel. They argue that jet fuel burns at a maximum of about 800 degrees Celsius. To melt steel, at least 1,500 C is required. As a consequence, they argue, fire could not have caused the collapse.

But steel doesn't have to melt completely. At 650 C, it loses 50 percent of its tensile strength. At just below 1,000 C, it loses about 90 percent, according to experts. Moreover, specialists believe that flammable materials inside - such as carpets, curtains, furniture and plastics - helped increase the temperature at the top of the towers to almost 1,000 C.

Why, despite being hit second, did the South Tower collapse first?

Although it was hit 17 minutes earlier, the North Tower remained standing for 29 minutes longer than the South Tower - for two reasons. As AA 11 impacted the North Tower 16 stories higher, there was less weight for the frame to support. And AA 11 was also flying more slowly and therefore caused less immediate damage.

Could the towers have been blown up?

Some critics still maintain that the Twin Towers could not have collapsed simply as the result of fire damage. They argue that the buildings must have been demolished. Indeed, television pictures do show minor explosions as the structures collapse.

The report issued by the National Institute of Standards and Technology (NIST) offered this explanation: Pieces of the planes sliced open the utility shafts inside the towers, enabling jet fuel to spread throughout the building. Explosions occurred whenever a spark ignited the fuel.

Beyond this, demolishing two buildings the size of the Twin Towers would have been a massive undertaking. With so many people involved, could it really have been kept under wraps?

Why wasn't anyone rescued from the roof?

Just minutes after the first tower was hit, police helicopters were already circling the World Trade Center. But smoke was streaming out of the North Tower, blanketing all but one small section of the roof. Additionally, the North Tower's roof was a dense forest of satellite and telecommunications masts, leaving no room for a landing pad. In theory, however, helicopters could have used rescue baskets.

The World Trade Center had already been the target of a terrorist attack in 1993, when police successfully airlifted dozens from the rooftops. But more than 10,000 had managed to escape via the staircases. Afterward, fire officials accused the police pilots of "showboating." Based on the experiences of the first attack, they argued it was more efficient to evacuate the building via the stairwells than to direct people to places where they would need to wait for help. Since the 1993 attack, massive steel doors had prevented access to the roof. An emergency services unit on the 22nd floor could have opened the doors, but communications with the top floor had been cut.

Helicopter rescues were impossible at the South Tower as well: the roof was completely obscured by thick smoke until its collapse.

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