The official version, the conspiracy theories and the evidence surrounding the 9/11 attacks.
Could the US Air Force have prevented the attacks?
To sceptics of the official account of 9/11, the idea of 19 fundamentalists hijacking commercial airliners and outsmarting the world's most advanced air defence system seems simply incredible.
Some 9/11 conspiracy theories argue that the US Air Force should have succeeded in intercepting at least some of the hijacked planes, and that someone, therefore, must have prevented them from doing so.
The official version - 9-11 Commission Report - holds that on the morning of 11 September, 2001 a major defence training exercise was taking place.
This exercise caused confusion, as some personnel initially thought the hijack reports were a drill. Even when the reports were confirmed, it still took nine minutes for the pilots to be scrambled. And once airborne, they were still unsure about where they were going: the fighters scrambled from Washington headed out to sea, rather than towards the hijacked airliners.
Davin Coburn, a reporter with the magazine Popular Mechanics, offers an explanation. The military, he argues, responded so poorly because they were simply unprepared.
A passenger airline had not been hijacked in the USA since 1979 - and all of a sudden there were now four at once with many other reports of suspect flights.
Also, the hijackers switched off the aircraft transponders - making the planes much more difficult to trace as they diverted from their scheduled flight paths.
But following the publication of the 9/11 Commission Report it emerged that the Commissioners were concerned they might have been deliberately misled by the military about the timeline of their response to events on the day.
Suspicions of a cover up by the military were recently addressed by a report from the Pentagon Inspector General.
The report found that there had been no deliberate attempt to mislead the 9/11 Commission, and that the discrepancies in their testimony to the Commission were the result of "a lack of capabilities and thoroughness" within the military.
Were the Twin Towers deliberately demolished by explosives?
After 9/11, investigations by the Federal Emergency Management Agency (FEMA) and the National Institute of Standards and Technology (NIST) determined that the collapse of the Twin Towers was due to the impact of the planes and the large quantities of exploding jet fuel released into the buildings.
Those questioning this account point to the lateral puffs of smoke that emerged from the towers just ahead of their collapse. Could these be explosive devices planted as part of a conspiracy?
They also argue that jet fuel, which has a far lower burning temperature than the melting point of steel, is unlikely to have weakened the steel supporting framework sufficient to prompt the collapse of the Twin Towers.
Jet fuel burns at 800 degrees Celsius whereas temperatures must reach 1,500 degrees Celsius for steel to melt.
The explanation for the puffs of smoke offered by the authors of the Popular Mechanics study is that as the floors crashed down of top of one another, a pressure wave forced dust and smoke out of the windows.
As for the fuel temperature - the official explanation holds that whilst steel does indeed melt at 1,500 degrees Celsius, it loses half its strength at a much lower temperature of 650 degrees Celsius.
The fuel might not have melted the steel columns, but it weakened the structure, and especially the trusses that supported each floor, to the point that they could no longer support the weight on the building.
Was WTC7 deliberately demolished by explosives?
In the afternoon of 11 September 2001, World Trade Centre Building 7, a 47 storey office block close by the Twin Towers collapsed without even being hit by the planes.
The building had been evacuated and there were no casualties and with so much else happening that day, its collapse was barely reported.
WTC 7 was home to local offices of the CIA, Department of Defense, the United States Secret Service and the city's Office of Emergency Management, among others.
Sceptics of the official account, including those at Scholars for 9/11 Truth argue that the building was deliberately destroyed in a controlled demolition, perhaps in order to conceal important information about a pre-9/11 plot by the authorities.
The collapse of WTC has been investigated by FEMA. Their interim report found that when the North Tower collapsed, debris crashed into Building 7.
This was the likely cause of fires which quickly took hold. The sprinkler system did not work effectively because the water main in Vesey Street had been knocked out when the Twin Towers came down.
With the intense fires burning unabated, the steel structure supporting the building was fatally weakened. But the FEMA investigators conceded that this hypothesis had a low probability of occurring.
In their final report, due to be published later in 2007, FEMA is expected to back its original hypothesis substantially - the collapse of WTC7 was accidental, not deliberate.
Were Jews forewarned about the attacks?
Shortly after the attacks a rumour started in the Middle East and spread around the world which claimed that 4,000 Jewish employees at the World Trade Centre had not turned up for work on 11 September. Were they warned to stay away?
One conspiracy theory suggests that 9/11 was an Israeli plot to discredit the Arab world; another that the Israeli intelligence agency, Mossad, found out about the attacks beforehand and warned the Jewish community in New York.
The rumour started after the Jerusalem Post reported that roughly 4,000 Israelis were believed to be living or working in New York and Washington.
Crucially it did not say they were dead or missing - just people in the immediate areas of the attacks who might have been affected.
This report was picked up by Arabic media outlets, including Al Manar, the Beirut satellite TV station linked to the Islamic militant group Hezbollah.
Al Manar TV added a spin to the story. It reported that 4,000 Israelis working at the World Trade Centre had not shown up for work on 9/11. As the anti-Semitic rumour spread, the details became embellished - like in a game of Chinese whispers.
According to official figures, of the 2,749 victims of the World Trade Centre attacks, 2,071 were occupants of the buildings.
Research by The Conspiracy Files shows that of these 2,071 victims, 119 were confirmed Jewish and a further 72 were believed to be Jewish.
This would make a total of 191 - or 9.2% of victims. This figure is broadly in line with the 9.7% of New York's commuting population which is believed to be Jewish.
Did a commercial airline hit the Pentagon?
At 09:37 on 11 September, the Pentagon, headquarters of the American military, was rocked by a huge explosion.
According to the official account American Airlines flight 77 crashed into the building, killing all passengers on board and 125 military personnel and civilians on the ground.
But some people argue that AA 77 could not have hit the building, as there is little visible wreckage, and the hole in the wall caused by the impact is too small to have been inflicted by a Boeing 757 airliner.
The first photographs taken before the front wall of the Pentagon collapsed show the hole appears to be some 18-20 feet across at its narrowest point.
Conspiracy theories argue that a remote controlled drone or missile struck the Pentagon.
The official explanation is that the fuselage - which is 12 feet wide - punctured the building, but the two engines and the wings largely broke up on impact.
US Defense Department photographs taken shortly after the attack show fragments of aircraft wreckage, some with the distinctive American Airlines livery.
The black box recorders and engine parts were found amongst the wreckage, and many eyewitnesses described seeing the plane hit the building.
Engineers and computer scientists at the Rosen Centre for Advanced computing at Purdue University, Indiana have built a computer model to recreate the crash.
Their research suggests that it was the exploding jet fuel which caused the worst damage inside the Pentagon.
Can CCTV footage prove what happened at the Pentagon?
Conspiracy theorists argue that the Pentagon, America's military headquarters, was not hit by a commercial airplane.
They say the initial hole in the outer wall is too small and there is no evidence of visible wreckage. Instead they believe the Pentagon was hit by something smaller, such as an unmanned drone or a cruise missile.
For Professor Jim Fetzer, of Scholars for 9/11 Truth, the evidence is clear-cut. "Whatever hit the building was not a Boeing 757," he told The Conspiracy Files. "Anyone that tells you a Boeing 757 hit the building is saying something that is probably false, that cannot possibly be true."
The Pentagon and the FBI have released some footage taken from CCTV cameras in the vicinity of the Pentagon.
Some of that footage - from the nearby CITGO petrol station and the Doubletree Hotel - does not shed any light on what hit the building.
Frames taken from two cameras at a car park in front of the Pentagon appear to show the silver nose cone and tail fin of a Boeing 757 moments before the explosion, but the footage is of very poor quality and not conclusive.
The FBI will not confirm whether or not it holds other security camera footage of the attack. Its lack of openness has fuelled further conspiracy theories.
There are many reports that security cameras at the nearby Sheraton Hotel captured images of the attack, but the hotel manager told The Conspiracy Files that none of their cameras were pointing in the direction of the Pentagon and no such footage exists.
The Conspiracy Files interviewed rescue workers who said they clearly identified wreckage from a passenger jet in and around the building. And photographs taken in the immediate aftermath of the explosion and shown in the documentary, identify parts of a Boeing 757 including a piece of fuselage with the distinctive livery of American Airlines.
Did a military transport plane control the attack on the Pentagon?
As the hijacked American Airlines Flight 77 approached Washington DC, a military transport C-130 aircraft took off from Andrews Air Force Base, en route to Minnesota.
Conspiracy theories suggest that the C-130 may have been controlling the attack that morning as part of a secret government plot, or even fired a missile which downed the Boeing.
The C-130's pilot, Lt Colonel Steve O'Brien, tells a different story. He told The Conspiracy Files that he and his crew were alerted by Air Traffic control to the presence of an unidentified jet approaching the capital.
O'Brien describes seeing a Boeing passenger jet with the distinctive silver and red colours of American Airlines crossing their flight path in an unusually steep bank.
His first reaction was that the aircraft was in trouble and trying to make an emergency landing at the nearby Reagan National Airport. Moments later the Boeing crashed into the Pentagon.
Lt Colonel O'Brien points out the C-130 is not capable of carrying missiles.
Was United 93 shot down?
United Airlines 93 was the fourth plane hijacked on 9/11, and the only one not to reach its target.
The official account of the day, as told in the 9/11 Commission Report holds that Flight 93 crashed into open ground near Shanksville, Pennsylvania, apparently as passengers tried to seize back the controls from the hijackers. But 9/11 conspiracy theorists are suspicious of this account.
Some argue that substantial wreckage from the aircraft was found at Indian Lake, a reported 6 miles from the crash site in Shanksville. If this would true, it would lend weight to the theory that the aircraft disintegrated in mid-air after being hit by a missile.
Leading conspiracy theorist and broadcaster Alex Jones of infowars.com argues that planes generally leave a small debris field when they crash, and that this is not compatible with reports of wreckage found further afield from Shanksville.
The reports of wreckage at Indian Lake were accurate in so far as small, light fragments of insulation material and paper from United 93 were found by residents at the lake, having blown there from the crash site on the prevailing wind.
However, the distance between the two locations was misreported in some accounts. In fact, in a straight line, Indian Lake is just over a mile from the crash site. The road between the two locations takes a roundabout route of 6.9 miles - accounting for the erroneous reports.
Also, the fragments of debris which were found at Indian Lake were downwind of the crash site and east of the plane's flight path. Had United 93 started to disintegrate in mid-air, wreckage would have fallen below the flight path - to the west of Shanksville.
Wally Miller, the local coroner at Somerset, Pennsylvania is at the centre of another debate about the crash of United 93. In his film Loose Change, Dylan Avery quotes Miller as saying: "I stopped being coroner after about 20 minutes because there were no bodies there."
However, interviewed for The Conspiracy Files, Wally Miller says he was misquoted.
"I said that I stopped being a coroner after about 20 minutes because it was perfectly clear what the manner of death was going to be.
"It was a plane crash, but yet it was a homicide because terrorists had hijacked the plane and killed the passengers."
He says it is technically correct that there were no complete bodies at the crash site, but the recovery operation found many body parts and DNA to identify all the passengers and crew on board.
Did United 93 crash?
Photographs taken at the crash site near Shanksville show a small crater and fragments of clearly identifiable aircraft wreckage along with personal possessions from some of those on board.
But the size of the crater and the absence of large pieces of wreckage have led some to question whether UA93 actually crashed there at all. This question is examined at the following website:
Dylan Avery, director of the hugely popular internet film Loose Change, argues that Flight 93 landed elsewhere, with the passengers abducted as part of an elaborate government plot.
Avery's film quotes reports from a local TV station in Ohio saying that two planes had landed at Cleveland Hopkins International Airport because of a bomb threat. Avery suggests that one of these was Flight 93.
In fact, there was one plane diverted to Cleveland that morning, with a suspected bomb on board.
But this was a different flight: Delta Airlines Flight 89, which had been proceeding along the same westward air corridor as United 93. In the chaos of that morning, Air Traffic Control had confused the two flights and ordered the Delta to land.
Local TV stations covered a press conference by the local mayor in which he referred to the Delta flight landing because of a suspect bomb, but later amended their stories when it became clear that Delta 89 had not been hijacked.
Could the attacks have been prevented?
Were there chances to stop the attacks prior to 9/11? If so, was this failure deliberate?
The CIA knew that two al-Qaeda terrorists had entered the United States as early as January 2000. But the CIA never passed on this information to the FBI - so the FBI did not know to look for them.
The two future hijackers, Nawaf al-Hazmi and Khalid al-Mihdhar settled in San Diego, where they started to take lessons at a local flying school. They used their real names in official documents and one of them - al-Hazmi - was even listed in the local phonebook.
Had the CIA tipped off the FBI that two known al-Qaeda operatives had travelled to America, it would have been easy to track them down - and possibly the 9/11 attacks could have been thwarted.
But Dale Watson, who led the FBI's investigation of 9/11, told The Conspiracy Files that there was no deliberate CIA plot to keep information from the FBI.
He said: "In large organisations you do have breakdowns in communications at the lower end. There was never any top down orders either by the FBI or the CIA. And anybody that believes that - they're wrong."
After the attacks, government officials were summoned to give evidence before a Congressional Inquiry set up to investigate the intelligence failure before 9/11.
It seems that the CIA information about al-Hazmi and al-Mihdhar was "lost in the system".
But Co-chairman Senator Bob Graham told The Conspiracy Files of his frustration at the lack of co-operation from the FBI in that inquiry, and by the government's decision to censor over 30 pages of his report which related to Saudi Arabia.
"Within 9/11 there are too many secrets," he said, "and that withholding of those secrets has eroded public confidence in their government as it relates to their own security."
The Conspiracy Files: 9/11 was broadcast on Sunday, 18 February 2007 at 2100 GMT on BBC Two.