In the blood-stained history of terrorism there has never been a day as horrifying as September 11, 2001.
As anger began to replace disbelief across the world, there were inevitably more questions raised by yesterday’s devastating suicide attacks on New York and Washington than answers.
Here are the main questions raised:
:: Why did US security forces have no intelligence about the attacks?
One Arab journalist claims he was warned by followers of Osama bin Laden of a ‘‘huge and unprecedented attack’’ on US interests three weeks ago but did not take the threats seriously.
Abdel-Bari Atwan, editor of the Al-Quds al-Arabi newspaper, said he received a warning from Islamic fundamentalists close to Bin Laden, but dismissed it.
‘‘They said it would be a huge and unprecedented attack but they did not specify,’’ Atwan said.
‘‘We usually receive this kind of thing. At the time we did not take the warnings seriously as they had happened several times in the past and nothing happened. This time it seems his people were accurate and meant every word they said.’’
But in America, officials were already blaming a lack of intelligence for yesterday’s tragedy.
‘‘In my view, this has been an intelligence failure,’’ said Senator Pat Roberts, adding that the Senate Intelligence Committee had no indication the attacks were coming.
:: How were the hijackers able to breach security in such a spectacular manner?
Speculation is already rife as to how no fewer than four planes could have been simultaneously hijacked and, one by one, diverted on a devastating path of destruction.
Among the suggestions were that the terrorists responsible for crashing the two jets into the World Trade Centre were trained pilots.
‘‘People might have infiltrated into aviation systems years ago and then merely waited to ensure that all the attacks worked,’’ said Kieran Daly, editor of the Air Transport Intelligence Internet news service.
But other experts suggested that the planes’ pilots had already been murdered and their seats taken by highly-trained renegades before the tragedies.
Gene Poteat, president of the Association of Former Intelligence Officers, said: ‘‘They flew the planes themselves. No pilot, even with a gun to his head, is going to fly into the World towers.’’
A British expert in Internal Relations said that no fewer than 30 highly-trained renegades would have been involved in the extraordinarily well-synchronised series of deadly attacks.
Professor Peter Lawler, a senior lecturer at Manchester University, said: ‘‘Certain individuals clearly can finance and organise terrorist activities on a scale that is simply unprecedented. They have access to enormous intelligence that normally only nations have access to.
But their task may have been facilitated by domestic flight security, which is weaker in the United States than it is in Europe, where countries are criss-crossed by international flights, an expert said today.
Furthermore, the fear of hijacking had receded since the 1970s and early 1980s when it was a favourite tactic of terrorists, said Phil Butterworth-Hayes, civil aviation editor with the Janes information group.
The two factors combined helped the suicide hijackers who carried out the US attacks to execute their deadly missions.
:: At what point did the authorities in America realise that the planes had been hijacked?
Although the details of what American authorities knew and when will not be known for some time, nobody could have predicted that two jets were to be redirected into the twin towers of one of Manhattan’s most famous buildings.
The four planes were all hijacked shortly after take-off and their flights had been monitored, including the dramatic changes in their routes.
It has emerged that the Justice Department’s control centre was also alerted about the hijacking of the plane which then crashed into the Pentagon.
The husband of a passenger on American Airlines flight 77 received a call from his wife and immediately contacted the Justice Department, but nothing could be done to save his wife and her fellow passengers. They were all killed upon impact with the Pentagon.
American Airlines has since only confirmed that its planes were Flight 11, a Boeing 767 en route from Boston to Los Angeles, carrying 81 passengers, nine flight attendants and two pilots.
The second plane was Flight 77, which was a Boeing 757, operating from Washington Dulles to Los Angeles, with 58 passengers, four flight attendants and two pilots on board.
:: What was done to intervene and try to prevent the string of atrocities which later unfolded?
Although the flights of the four planes were monitored, no apparent move was made to intercept them.
But there was also speculation that pilots - perhaps flying at gunpoint - realised the full horror of what was going to happen and tried to do something to stop it.
This might explain why the United Airlines plane from Newark crashed into a wood in Pittsburgh, Pennsylvania, rather than a city landmark or - 85 miles away the President’s mountain retreat at Camp David.
Meanwhile, airports were closed, flights were grounded, borders were sealed and jet fighters were scrambled above major cities.