At first it looked like snow. There was a stillness in the city. White dust floated in the air. People looked up to see where it was coming from. The traffic was moving slowly. Then people started running, running north and running east. It was around 9 in the morning on Sept. 11.

The man who was recording the events with his video camera, Evan Fairbanks, had just emerged from Trinity Church, where he was videotaping the archbishop of Wales. He came out in time to photograph the paper snow. On the street, people were dialing their cellphones again and again to tell their someones the news. They were looking up at a white plume of smoke. Against a crack of blue sky just above the American Stock Exchange you could see the source of the strange weather. One of the towers of the World Trade Center was on fire. And things were remarkably calm.

Mr. Fairbanks has made a Zapruder film for our time. His camera is there but always seems just a second or two behind the events. He doesn't know what he is photographing. How could he? And he certainly doesn't know what it will look like in retrospect. He has no idea of the things that people will know when they finally do watch it.

His videotape, 25 stunning, silent minutes being shown on an endless loop at The New-York Historical Society, is just one part of the exhibition "New York Sept. 11" by Magnum Photographers. As it happened a lot of photographers from the Magnum agency were in New York that day, including Paul Fusco, Thomas Hoepker, Steve McCurry, Susan Meiselas, Gilles Peress, Larry Towell and Alex Webb, because their annual meeting was the night before at the agency's office in Chelsea. The bulk of the exhibition at the historical society, on view until Feb. 25, is their photographs and written reminiscences of the disaster, as well as some pictures of the World Trade Center taken long before Sept. 11

Mr. McCurry captured the plumes and geysers from the crashing towers and then the terrible aftermath. One of his pictures shows a line of firefighters, with yellow stripes on their jackets and pants, threading their way through the new gray world, clutching a long fire hose like children linked together on a leash for a school trip.

Ms. Meiselas caught the stampede of frightened people running through the streets as the cloud of one collapsed building thundered close behind. Mr. Webb caught masked Manhattanites in the rubble. Mr. Hoepker captured the golden and black sky over the Brooklyn Bridge. Mr. Towell photographed a dazed man in the desolate street reading a single sheet of paper from the debris as if it were poetry. Mr. Peress caught the dust.

The Magnum photographers' still pictures are awful in their gorgeousness. But they cannot compare with Mr. Fairbanks's shaky videotape. And there is a simple reason.

Mr. Fairbanks, who is not a Magnum photographer, caught the events before anyone knew what they would mean. He saw the very climate changing minute by minute. His tape is not only a record of the events unwinding; it also defines the moment just before everything changed: firemen, police officers, cellphones, office papers, orderly evacuations, flags, the American Stock Exchange, blue skies.

The video shows people pointing up at the sky after the first building has been hit. A New York City Fire Department ambulance rumbles down the street. One man in a T-shirt is outside Steve's Pizza yelling and gesticulating madly, showing with his hands what the fireball looked like when the plane hit. He looks oddly out of place, like a street preacher talking about the end of the world.

Some people are smiling, the way people might smile at a monster truck rally. People are leaning on cars. They are getting comfortable with a new, slightly modified reality. Looking up toward the World Trade Center, you can see that it is wounded, but certainly not mortally. An F.B.I. agent is writing something on the hood of a car. A black ribbon blows in the air. It isn't yet an omen.

Then a long, long moment changes everything. Over the head of the F.B.I. agent, who clearly does not see what is happening, a plane silently penetrates the other World Trade Center tower. The man's head reels out of the frame as he reacts to the crash. His head snaps back in time to watch the aftermath. A black cloud envelops the tower. Debris sprays out like a fountain from the top. The sky goes dark. The traffic stops.

And the weather shifts completely. It is no longer a gentle snow, but a tornado outside. Papers the size of newspapers are blowing everywhere. A man's shadow runs and throws down a briefcase near a wall. The camera gazes at the wall, then focuses on a cellphone. The neighborhood is dark and deserted.

Firefighters start marching toward the tower with oxygen tanks. The street sign blinks "Don't walk." They are marching very, very slowly. You wish they would go faster — or stop. Some look worried. But what do they have to worry about? The buildings they are marching to still look sturdy. The camera follows them into the World Trade Center. People are emerging from the bottom of the building without panic. They wave to their friends. The sun streams in. Everything will be O.K.

Then the camera goes down to the Emergency Control Center in the basement of the World Trade Center. The room is ordinary, ugly and bureaucratic looking. Men stand around arguing about strategy, as if it were just another power struggle. There is an American flag on the wall. The camera and the cameraman leave the building for the last time. The camera catches sight of the Port Authority police station. The black cloud at the top of the tower begins to descend, faster faster faster. The world shakes. And everything goes black.