The first bang came at 8.50 a.m., shaking the windows of my 42nd floor office which has a clear view of the two World Trade Centre towers three blocks to the south-east.
I looked out and saw a huge ball of flame and black smoke billowing out of the north-facing side of the nearest tower.
As the smoke cleared a massive hole 10 stories high became visible just below the top 10 floors and flames could be seen encircling the building behind its narrow slit-like windows.
Ten minutes later a passenger plane appeared from across the Hudson River heading straight for the second tower.
I didn't notice it until the last minute.
It was tilted so that the flight path took it straight towards the second tower.
It hit and simultaneously a gigantic ball of flame emerged from the east side of the second tower as if the plane had crashed right through the heart of the 110-store building.
Debris fell in chunks onto West Side Highway followed by a blizzard of shards of glass and paper where ambulances and fire engines were beginning to congregate.
Flames began to leap from the side of the Marriott Hotel just across from where the tiny white Greek Orthodox church stands incongruously in a car park.
People by this time were streaming from the bottom floor of the World Trade Centre, which stands on top of a shopping mall and wide marbled corridors containing airline offices, shops, and a subway station.
Some 50,000 people work in the seven-building complex and many would have been in their offices by now.
Tourists would have begun arriving to queue for the lifts to the top of the tower with its famous window on the World Restaurant and wonderful views over Manhattan.
But they would not have been taken up to the 107th floor observation deck until it opened at 9.30 a.m.
Watching through binoculars I could see people hanging out of the windows beside and above where the fires were raging.
One man waved a white cloth as he clung to a window strut.
Then a body fell from a window above him with arms and legs outstretched and plummeted some 100 stories onto Vesey Street.
Two other people fell in the succeeding minutes.
I ran down to the streets where office workers and traders, still wearing their red jackets, were milling around.
Some women were screaming "Oh my God, people are jumping, Oh my God."
Some of the residents of my building, where many members of the financial district live, were sobbing uncontrollably. I returned to the office.
As I watched, the top of the second tower suddenly fell outwards onto West Side Highway, the main thoroughfare along the western side of Manhattan which passes right by the World Trade Centre.
Massive jagged pieces of the tower the size of houses crashed onto two fire engines and onto rescue workers on the roadway.
A huge cloud of dust and ashes rose from the impact, enveloping the Embassy Suites Hotel across the highway and the 50-storey buildings of the World Financial Centre which house Merrill Lynch and stands between the towers and the Hudson River promenade.
I shifted my eyes upwards to the first tower that had been hit and was still standing, and saw that several more people had appeared in the upper stories where they had smashed windows.
The man with the white cloth was still there, hanging precariously by one hand with his body out over the abyss.
I wondered why there was no attempt to rescue them by helicopter as part of the roof of the 1,350-foot building was clear of smoke.
But then the tower began to sway slightly and two people fell in quick succession from the windows as if unable to maintain their grip, falling down onto West Side Highway into the dust and smoke from the first collapse.
Then the tower simply slid in on itself, imploding with a huge roar, leaving the lift-shaft like a stump of a blasted tree with twisted metal arms.
This time the clouds of dust and smoke were so huge they enveloped the whole of southern Manhattan.
Thousands of tons of rubble fell onto Tobin Plaza, where open air concerts are sometimes held and onto the annex housing Borders giant book store.
A westerly breeze kept it about a hundred yards from my building. As it cleared, the scene around the towers was like one from a war zone, which it truly was.
The roadway and pavements and bicycle path, all the streets, cars, fire engines, pavements, traffic lights, awnings and police vehicles, were coated with dust. The green park between my building and the towers where kids play American football had been transformed into a grey filed, as if covered with a toxic snow.
About 20 cars in the open air car park between it and the disaster scene were on fire.
As the dirty grey and brown smoke cleared around midday I could see that the Marriott Hotel had taken the full force of the falling tower.
All its windows were shattered and long strips of jagged metal hung over the awning.
The wide, covered pedestrian bridge from the mezzanine level of the financial centre and the Winter Gardens, with its palm trees, was a twisted mass of wreckage.
Out on the streets again, people who had been standing aghast in Battery Park by the river were now streaming uptown on foot.
"Go north, head north," shouted a police officer. The police and firemen remained mostly calm but their faces reflected the horror of the certainty that they had just lost many comrades.
Before the collapse dozens of firemen and police had rushed into the twin towers to try to help with the evacuation.
What had become of them? It was too horrible to contemplate.
Or to think of the thousands of people trying to escape from both towers, executives in suits, receptionists at polished desks, secretaries, traders, messengers, choking in their offices or racing down smoke-filled emergency stairwells below the destruction line.
It was just 10 minutes before nine when the first plane crashed into one of the towers and the offices were undoubtedly all either doing business or preparing for another working day.
The two gift shops at the top and the food court would have been almost ready to open up.
In the mall below thousands at that time of day criss-cross through the wide corridors past designer stores and coffee shops and newspaper vendors.
Shocked and weeping people trotted down Greenwich Street and west Broadway and Broadway with their hands to their eyes, obeying the arm-waving police officers, and fleeing in the direction of uptown.
Others crowded onto the cross-Hudson ferries at the harbour surrounded by the financial centre buildings.
The ferries had borne workers from the New Jersey shore to their work in the financial district just two hours before. The clothes of those caught in the explosion of dust were coated with grey, some had dust-covered hair and eyebrows.
Between midday and one o'clock there were three more muffled bangs from the vast area still hidden by thick acrid smoke as more sections of the two towers collapsed.
As the smoke drifted eastwards further scenes of immense destruction came into view: The twisted metal and piles of rubble and crushed dust-shrouded emergency vehicles which filled Vesey Street and West Side Highway.
There among the rubble, coated with dust, were many bodies, some of those who had jumped, and some of firemen caught beneath the heavy, deadly cascade of falling concrete and metal.
Another plane appeared overhead, causing a frisson of panic but it turned out to be a military jet, arriving one assumes to shoot out of the sky any further suicide pilots. It was too late to save downtown Manhattan.