Air traffic controllers at LaGuardia Airport in New York first noticed something was amiss when, looking through binoculars, they saw a plane plunge into the World Trade Centre.
At the New York Air Route Traffic Control Centre in Queens, which handles all long-distance air traffic in the area, the first indication something was wrong came when a supervisor suggested that controllers turn on CNN.
How could this have happened? Not only are people asking how could it have happened in New York, but how could a plane have flown uninterrupted into the Pentagon, nearly an hour after two planes crashed in New York when it was obvious the US was in the midst of a terrorist attack?
Even as rescuers search through the debris trying to find survivors, attention is turning to what many are calling a massive failure of US security.
The blame can be attributed widely.
Despite spending $30 billion each year on intelligence, the US had no clue a massive terrorist plot was afoot in the last year. The FBI now says at least 50 people were involved. They have positively identified the names of 19 hijackers.
The men, by all accounts, lived openly in various cities in Florida. They travelled freely from Canada, moving across the largest undefended border in the world, drove to Boston and Los Angeles in rented cars, stayed in hotel rooms, graduated from various aviation academies. One man's tuition at a flight school in Florida was paid for openly by the government of Saudi Arabia, a not unusual arrangement, according to the head of the school.
Several of the men were known to frequent bars in Vero Beach, Florida, and a waitress was quoted as saying they drank too much. A neighbour said one of the men had a poster of Osama bin Laden in his apartment.
But still US intelligence had no idea they were in our midst.
Furthermore, there is the matter of the 18 men who boarded four different planes in three different cities on Tuesday. With apparent ease, they stabbed flight attendants and took over the cockpits of each plane.
With the easy flip of a switch, they turned off the transponders (the robot radios on a plane's tail that give information on location to ground-based radars) on three of the planes, making in more difficult to track them. On United Flight 175, the second plane to hit New York, they changed the code on the transponder, confusing the controller's computers.
Still no one on the ground panicked.
One controller told the New York Times that it is standard procedure to do nothing if a pilot strays from his flight path; standard procedure is to "give him room and watch what he does," he said.
But what about the Pentagon? The airspace is protected there. No one is allowed to fly below 5,000 feet. And yet the US military did nothing as the plane entered the protected airspace. The pilot skilfully hit the buildings.
David Russell, a Vietnam veteran and a pilot, said the rogue pilot was no amateur.
"The World Trade Centre is one thing; it's 110 stories and you just kind of aim for it. But the Pentagon is a tough building to hit. It is only five storeys high. This pilot showed great skill in hitting that building."
What emerges is that there are no procedures in place to deal with such scenarios, a fact Americans find shocking. The US military will not shoot US commercial planes out of the air. The pilots who commandeered the planes knew they were in no danger of that.
Back in 1978, the Federal Aviation Administration (FAA) found that airport screeners missed guns and pipe bombs brought onto planes 13 per cent of the time. In 1987, the FAA found that screeners missed weapons 20 per cent of the time. The agency then stopped releasing the results of its compliance tests.
Screening and security in the US is handled by four private companies, three of which are European owned. In Europe, the companies pay screeners about $30,000 a year. In the US, they are paid about $15,000 a year, or about $6 to $7 an hour.
David Stempler of the Air Travellers' Association put it aptly: "The people are poorly trained and at that wage would normally be flipping hamburgers." One of the companies, Argenbright, last year pleaded guilty to falsifying records and doing inadequate background checks on screens. They had hired at least 14 workers with criminal records for drug dealing, burglary and illegal firearm possession. The company paid a $1.2 million fine.
The vulnerability, ineptitude and absence of preparedness by a range of government agencies is something Americans are slowly coming to terms with.
Here there is a unified cry for war. The New York Post, in a lead editorial, said "Bombs away" and called for a broad attack on the countries who harbour terrorists. But it may take some time for Americans to realise that its military might not be ready for that.
During the war in Kosovo, many wondered why the US would not commit ground troops. One NATO officials put it succinctly back then: "We don't do mountains," he said, referring to the fact that the US military has prepared over the last 10 years for desert-based conflict, not for mountainous terrain.
This week, new security procedures for US air travel were announced. No more curbside baggage check-in and no more steak knives during meal service.Such rules are a joke to the people who run security at Israel's El Al Airlines.
They retrofit the cockpit door of their US-made jetliners to make them impregnable. And the doors are never opened during flight.
Never.