As the United States government pressed ahead with efforts to track down the perpetrators, ordinary citizens around the world were assessing their own response to the catastrophic acts of terrorism that took place in the United States on Tuesday.
From rooftops in Brooklyn and Manhattan, from living rooms where the crashes of hijacked airliners were replayed on television, the world had a front row seat to disaster. Before their eyes, the World Trade Center towers erupted in flames, and crumbled, after two planes struck them. Smoke poured from the Pentagon.
Wil Kamp, an architect who works for the Lawrence Group in Manhattan, was among the millions who watched the deadly events unfold. He saw them from the window of his own apartment in lower Manhattan.
Kamp was readying for work, he told swissinfo in an exclusive interview, when he looked out the window and saw flames erupt from one of the World Trade Center's twin towers. He thought a small plane had veered into it.
Stunned, he watched as a commercial airliner struck the second tower. Then, from the window, he and his wife watched the further stages of the unfolding catastrophe. Workers trapped high in the towers leapt to their deaths.
"The tower was gone"
The couple climbed to the roof of their apartment building, where they watched the American financial landmarks disintegrate.
"When the first tower collapsed, we didn't know what had happened because there was so much smoke and then when we saw the blue sky behind it, we realised that the tower was gone," Kamp said.
The horror overcame them as explosions boomed on lower floors of the trade center. Columns in the middle of the buildings caved in first - they were weaker than those in the foundation.
Kamp's wife became inconsolable. "She was hysterical," he said.
It was the realisation of the cataclysmic deaths of thousands of people and the destruction of the city's familiar skyline - the devastation.
"Just imagine, it was like the Eiffel Tower coming down and then compounded by 30,000 people dying. It was surreal. You couldn't believe something that big was happening in front of your eyes in a matter of an hour."
A surreal streetscape
As friends telephoned, he learned that several handicapped people in heavy electric wheelchairs had been stranded in dark stairwells of damaged trade center buildings, as the fire set off sprinklers, which made the stairwells slippery, a problem compounded by ash and dust from the fallout of the blasts.
In the long hours that followed, out on the streets New Yorkers seemed to be walking in a state of shock, he said.
"You'd see this surreal streetscape where you'd see people walking with their strollers and as you pan up, over the low rise buildings, you'd see this plume of smoke going across the entire skyline."
The absence of traffic was "eerie" in what had been a bustling section of the metropolis - there was none of the usual sound of aircraft - except for the buzz of F-15 military jets circling overhead. Regular broadcast television wasn't heard, because the trade towers no longer existed to handle transmissions. People switched to radio or cable channels.
Shops seemed stopped in time and not even newspapers were permitted into the cordoned off section of the city, where only residents and emergency workers were allowed.
Kamp saw his fellow New Yorkers lining up to donate blood at the St Vincent's Hospital nearby. The hospital is also treating a number of people wounded during the attacks.
He felt the isolation brought on by the terrorist acts: bridges and tunnels in the city are currently closed and rail services have been cut back.
Many people are in suspense, waiting to hear whether they recognise any of their friends or relatives' names among the dead. It's not just a tragedy for the United States, Kamp observed. He thought of all the people from other countries who worked at the World Trade Center. Tuesday's attacks are a tragedy for the rest of the world too, he said.