Conspiracy theories such as those popularized in the Internet documentary Loose Change are all the rage. Yet they are easy to refute, using new evidence from video and audiotape recordings, statements of captured al Qaeda members and the reports of commissions investigating the events.

First, there were warnings: In July 2001, an FBI agent in Phoenix, Arizona, fired off a memo to two of the bureau's anti-terror experts, one based in New York, the other at headquarters in Washington. The agent expressed concern that Osama bin Laden was sending students from the Middle East to take aviation lessons in the United States. He recommended that a list of flight schools be compiled and their attendance records examined to determine which foreign students had signed up for courses.

The memo disappeared into a sea of red tape, only to resurface after the damage of September 11 had been done.

At the end of June 2001, the CIA warned that a "major strike," a "spectacular attack" could be in the works. There were hints, scraps of information and an assortment of clues, but no one who could piece together this deadly mosaic.

Intelligence and communications failures, particularly between the FBI and the CIA, are detailed in the 9/11 Commission Report, which runs to nearly 600 pages.

The U.S. Federal Emergency Management Agency (FEMA) has issued its own report about rescue operations at the World Trade Center (WTC) and the Pentagon. And the National Institute of Standards and Technology published an analysis in 2005 that explains the causes of the towers' collapse.

Many of the questions posed by 9/11 can be answered more thoroughly and precisely today, allowing quite a few of the conspiracy theories to be debunked.

For example, information made public in the spring of 2006 clearly shows that the fourth hijacked airliner, United's flight 93, was not shot down by U.S. fighter jets, notwithstanding the numerous Internet rumors. At the trial of the suspected terrorist Zacarias Moussaoui, the prosecution played the audiotape from flight UA 93's black box. It is a dramatic recording, but there is no evidence that the plane was shot down.

In this sense, 9/11 is no different from the assassination of John F. Kennedy, the fatal car crash of Princess Diana and Dodi Fayed or - for Germans - the suicide of politician Uwe Barschel. Conspiracy theories abound. Not everyone is willing to swallow the official version of events. Some are convinced that information is being suppressed - they allege cover-ups, secret plots. They seek to implicate the government. They chat away in the blogosphere. And every attempt to refute a theory by analyzing fresh information spawns a new generation of suspicions.

Still, U.S. officials have done their part to abet the conspiracy theorists. Until recently, when they released the entire film, only five still surveillance photographs of the plane hitting the Pentagon were in circulation. At first, the torrents of debris unleashed by the collapsing towers of the World Trade Center were described as harmless. Today, New York doctors are treating patients with pulmonary ailments accompanied by a serious, hitherto unknown type of cough.

The substantial amount of new information available now, five years after the attacks, is inviting a re-examination of the events of the day. Some findings merely confirm what we already knew, but add layers of fresh detail. Others shift the focus, including the exact role played by Osama bin Laden. And much is new, including the dramatic final minutes of UA 93.

How did the hijackers get past airport security?

Nine of the 19 hijackers aroused suspicions during airport screening. In the end, they were waved through. And even if their knives and box cutters had been discovered, passengers were permitted to carry blades measuring four inches or less at the time.

On September 11, 2001, airports were using a computer system called Capps. Its job was to automatically identify potentially dangerous passengers. But the system wasn't equipped to deal with the likes of the 9/11 hijackers. At the time, security officials assumed that the main threat came from terrorists who would plant a bomb on a plane but not board themselves. Capps was programmed to detect people who survived the attack - and subsequently needed to cover their tracks. One of Capps' methods was to identify passengers who had paid for their tickets in cash, rather than by credit card.

Two of the five hijackers on American Airlines flight 77, which crashed into the Pentagon, were flagged by Capps. An airline employee at the check-in counter logged a question about two others into the computer: one of the al-Hamsi brothers had an ID with an error, and both were exhibiting suspicious behavior. But none of the suspects underwent further security checks; that's not how the system worked. Instead, airport personnel had to ensure that the luggage was only loaded onto the plane after its owners had boarded. Once a passenger had taken his or her seat, officials assumed the individual was not a terrorist.

The metal detector did sound on three of the five hijackers on AA 77. Two of them were sent to another detector and then subjected to a personal security check with a handheld device. One security employee even performed a wipe test on Nawaf al-Hamsi's shoulder bag, looking for traces of explosives. None were found, and he was waved through.

Were the 19 hijackers on the passenger manifests?

One of the most abiding 9/11 myths centers on the fact that the passenger manifests for the four airliners (first broadcast by CNN) didn't contain a single Arab name, implying that the 19 suspected hijackers weren't on board. As a result, they couldn't have carried out the attacks.

In fact, though, CNN had only broadcast the names of the victims - the passengers and flight crew, not the names of the terrorists.

On September 13, 2001, the Boston Globe obtained a complete list of passengers for flight AA 11, which rammed the North Tower. Mohammed Atta was in seat 8D - in business class. Sitting across the aisle were Hollywood producer David Angell and his wife, Lynn, in seats 8A and 8B. The hijacker Abd al-Asis al-Umari sat in seat 8G, next to Atta. As the Globe's investigative report showed, the seats occupied by the other hijackers on the three other planes have been long known. Additionally, ground staff were able to identify most of the men.

Who was the missing 20th hijacker?

For a long time, the U.S. government had suspected the Frenchman Zacarias Moussaoui, who was recently sentenced to life imprisonment for his alleged role in planning the attacks. This June, a website with ties to al Qaeda floated its own version, claiming that a Saudi named Turki al-Muteiri was to have taken part in the kamikaze mission. He wasn't on board any of the planes because the date for the attack was brought forward. Muteiri died during a 2004 gunfight in Riyadh.

Is Mohammed Atta's will authentic?

Yes. Abdelghani Mzoudi, one of the Muslims who witnessed the will, confirmed its authenticity to SPIEGEL. Suspected of aiding the terrorists, he was tried by a Hamburg court but acquitted. He then left Germany for Morocco.

Have the hijackers been positively identified?

Three of the hijackers on board the two planes that hit the World Trade Center have been positively identified. The FBI collected enough evidence from their rental cars and hotel rooms to create DNA profiles of all 10. Because the FBI coded these DNA profiles simply as "K" ("knowns"), medical examiners were able to positively identify the hijackers, but unable to attach names to the remains.

The FBI has not disclosed which of the 10 hijackers have been identified. Robert Shaler, the head of the medical examiner's team, believes that the three must have been in the rear of their plane when it hit the tower. "I still doubt the pilots have anything remaining to collect or analyze. Likely they were vaporized along with many of the innocent victims," he has written.

The nine hijackers on the other two planes have been identified as well - not by DNA analyses, but by a process of elimination: DNA was obtained for the crew members, Pentagon employees and other passengers (using toothbrushes etc. provided by relatives). Anything that couldn't be matched with these samples was assumed to be from the hijackers.

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.

How many people jumped?

Some leapt holding hands. Others fell alone. The desperation that made these people choose certain death is beyond imagination. Out of respect for the victims, many publications elected not to print images of the "jumpers." As a result, a sound recording remains the most chilling document of their deaths. A cameraman in the lobby of the World Trade Center filmed the horror-stricken expressions of the firefighters while his microphone picked up the dull thuds of the bodies hitting the ground.

In 2004, in an attempt to ascertain how quickly the fire and smoke had spread, NIST experts using video footage determined when and where people had leapt from the building. According to this and other studies, more than 200 jumped to their deaths.

Was the debris examined?

One tiny shard of glass about the size of a matchbox is all that remains of the Twin Towers' 43,000 windows. The rest were vaporized or pulverized. The collapse turned the two towers into 1.6 million tons of steel, rubble and dust. In an operation lasting months, the debris was shipped from Manhattan to New Jersey and deposited at a former landfill called "Fresh Kills." Piece by piece, rescue workers and investigators scoured the site for clues - body parts, fragments of bone, teeth or personal items. The energy unleashed by the collapse of the towers is illustrated by the discovery of "meteorites": blocks a full meter thick containing concrete, iron rods, pieces of furniture and steel matting from the buildings' flooring - all melded together by the heat and pressure.

After they were analyzed, the towers' girders were cut up and recycled around the world. Steel from the World Trade Center may now be in currency as parts of bridges, cars and cans.

How toxic was the cloud of debris?

In January 2006, New York police officer James Zadroga died of respiratory failure due to ''granulomatous pneumonitis,'' or fatal scarring of the lungs by hazardous material. Part of the 9/11 rescue effort, Zadroga was the first person whose death was officially linked to the debris and smoke at Ground Zero. The asbestos from the towers, pulverized cement and fiberglass, dioxin, PCB, the toxins from 50,000 vaporized computers, along with the diesel fuel from the excavators, have left some 15,000 people suffering from respiratory problems or the so-called WTC cough. A class-action suit has been filed against the Environmental Protection Agency (EPA) by local residents and the parents of schoolchildren: shortly after the attacks, its director had suggested that residents return home and schools reopen, arguing that the air in Manhattan did not represent a health risk.

Additionally, a suit against the City of New York has been brought on behalf of 8,000 firefighters who claim they were given inadequate breathing equipment for use at the World Trade Center site.

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