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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.

Were the flight data recorders of the WTC aircraft ever recovered?

The 9/11 Commission Report puts it succinctly: "The CVRs and FDRs from American 11 and United 175 were not found." Even before the report's release, experts expressed doubts about the recorders' ability to survive the crash into the towers, the ensuing fire, and finally the buildings' collapse. But thanks to statements made by firefighter Nicholas DeMasi, the whereabouts of the flight data recorders remains one of September 11's unresolved mysteries. In his 2003 book, DeMasi claimed he found three of the four black boxes at Ground Zero during October 2001.

DeMasi's assertions gained further credibility in December 2005 when the magazine CounterPunch quoted an anonymous source from the National Transportation Safety Board (NTSB): "Off the record, we had the boxes," the source said. "You'd have to get the official word from the FBI as to where they are, but we worked on them here."

Unfortunately, the source's identity remains a secret. But it is unlikely that three of the four black boxes would have been found during a single search at a time when the rubble of Ground Zero was still smoldering.

To this day, the FBI and the NTSB deny that the boxes were recovered.

Who was the woman in this picture?

The photograph shows the area where the plane struck the North Tower. A woman is visible near the bottom edge - on about the 97th floor. She is holding on to a severed pillar, waving. The picture conveys the utter despair of those trapped inside.

Numerous websites identified the woman as Edna Cintron (46), an employee with the insurance brokers Marsh & McLennan. Cintron's husband reportedly confirmed her identity.

Because Cintron was standing at the point of impact, some bloggers have assumed that the heat couldn't have been sufficiently intense to melt the steel inside the building. Cintron's photo ostensibly supports the hypothesis that the towers were intentionally blown up.

This is, of course, nonsense. Telephone calls made by victims show that not all the offices on the floors hit by the planes were destroyed immediately. Nor was the temperature consistent throughout each floor. Some of the trapped reported that they had fled to cooler areas - and the areas where the fire was sucking in fresh air were coolest of all, i.e. at the points where the aircraft had breached the walls.

Most likely, hardly anything about this story is accurate, even the woman's name; there is no evidence to substantiate her identity. Her husband, who reportedly recognized her, has discussed his wife's death in several interviews without ever referring to the picture. None of her colleagues, including the company's own memorial site on the Internet, has mentioned the photo. The only certainty is that the real Edna Cintron was not among the survivors. Nobody knows if she is the mystery woman in the picture.

Why did WTC 7 collapse?

The 47-story World Trade Center 7 building burned for seven hours beside the rubble of the Twin Towers - before collapsing in just eight seconds at 5:20 p.m.

FEMA's 2002 investigation concluded that WTC 7 had sustained comparatively slight damage prior to caving in. The 9/11 Commission made no mention of WTC 7 in its findings. And when the authorities remain silent or withhold information, rumors are guaranteed. Didn't the structure collapse a little too neatly? Could it have been dynamited?

The collapse of WTC 7 is currently being reviewed by the National Institute of Standards and Technology. Its experts believe that the building suffered far more damage from the plunging towers' rubble than had previously been assumed, particularly on the south side, the upper floors and the southwestern corner.

NIST's experts also attribute the building's ultimate collapse to the unusually heavy load borne by its individual supports. The buckling of a single support could have brought the entire structure down. The institute has found no evidence to suggest that bombs, missiles or explosives caused the building to cave in.

The final report has already been postponed several times. It is now due to be released this year.

How did the plane fit through the hole?

The hole that the Boeing 757 drilled in the Pentagon's exterior wall was some 10 yards narrower than the airliner's width. Just how credible is the assertion that flight AA 77 really did crash into the building? And if the damage could not have been caused by a Boeing 757, could a cruise missile have been responsible?

Conspiracy theorists like the Frenchman Thierry Meyssan maintain that a silhouette of the smashed plane should have been visible on the building's façade - an assumption refuted by physicists, who say that the remains of the plane after impact would have acted like a solid object rather than a fluid. Eyewitness accounts reveal that part of the right wing came off when the plane struck a huge generator in front of the building. The plane's left wing was smashed by the façade's support pillars.

The cruise-missile theory is also discredited by the various eyewitnesses who saw a jetliner crash into the Pentagon. What is more, the remains of the plane's crew and passengers have been identified.

Another obvious question presents itself: If flight AA 77 didn't smash into the Pentagon, where is it today?


Where is the wreckage from flight AA 77?

Hardly any aircraft parts are visible on photographs of the crash site at the Pentagon - according to skeptics, further evidence that a plane didn't hit the Pentagon.

But eyewitnesses who rushed to the scene reported seeing sections of the aircraft scattered widely throughout the vicinity of the crash. These included the nose, a tire, parts of the landing gear and fuselage, a cockpit seat and even a chunk of the tail with the plane's number on it.

There is no secret about the fate of the wreckage. On September 12, the Washington Post reported that a column of 50 FBI agents had marched shoulder-to-shoulder across the south grounds of the Pentagon, "picking up debris and stuffing it into brown bags. The lawn was scattered with chunks of the airplane, some up to four feet across," the newspaper reported.

Both of the plane's black boxes were also recovered. And all but one of the passengers on board flight AA 77 were positively identified - using remains recovered from the scene.

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