That obstinate cop on the barricade at Fulton and Broadway probably saved my life. It’s less than a minute before the South Tower of the World Trade Center collapses on the morning of 11 September 2001. I’m a block away trying to get closer, waving my press card in the cop’s face. He stands his ground, barking: “It’s not safe, it’s not safe.”

Then all hell breaks loose. Someone pelts past us screaming, “It’s coming down”, as billows of dust and smoke punch high into the sky over the end of the street. Adrenaline kicks in and I start to run across Broadway and down Fulton, chased by the debris of the collapsing South Tower.

About 20 metres down I skid to a stop, thinking: “Shit, I’ve got to take a picture.” I spin around, grab one of my Nikons and position the shot. As people sprint towards me, I fire off 13 frames – among them the one that will become famous.

I run some more. Aware I can’t escape the advancing cloud, I pick up my camera again and photograph the debris as it whooshes past, blotting out the sunlight and plunging us into a silent, twilight world of lurching, dust-covered people and tumbling ash.

“This is it, I’m going to die,” I think as I struggle to breathe. That sends me into automatic mode. I’m a photographer for the Associated Press news agency. I’ve got to send my photos. I’m on deadline and the seconds are counting down like an incessant drumbeat.

I dive into the lobby of an office building where others are already taking shelter, but the noise of wailing and panicking voices unnerves me and I need to get out. I tie my cardigan around my face as a mask and head back out into the dust.

For a while I stumble around, dazed, inside the muffled quietness of the cloud, photographing others in the same state. At some point I call my dad, leaving him a voicemail to tell him I love him – it’s a message he later says he listens to again and again for weeks.

Twenty-five minutes have gone by since the South Tower went down and I’ve still got to send those photos. I squeeze through the door of a tiny shop farther down Fulton where about 15 other people are taking shelter. The owner locks the door behind me as other people hammer on the glass. We don’t know it, but the North Tower is now crashing to the ground.

Suzanne Plunkett in London last year. Photograph: Dan Kitwood/Getty images

The people in the shop crowd around as I hook up my laptop to my clunky Nokia and start transmitting my photos. September 2001 is still pre-iPhone, pre-Wi-Fi, pre-4G, even pre-3G, and sending images from a remote computer is a novelty. One woman giggles awkwardly as she spots herself in one of the photos.

In the AP office in the Rockefeller Center, my editors see the images appear on their feed. It’s the first contact they’ve had with me since the day before. Relieved I’m OK, they call and ask if I can head to an apartment block overlooking the site of the towers to take an overhead shot.

On a balcony about 20 storeys up, the turmoil is still unfolding beneath. More buildings are groaning and collapsing, throwing new clouds up into the blue sky. Sirens blaze through the streets.

Inside the apartment, a three-year-old watches The Lion King on TV, the volume on high. His mother is on the phone complaining about her broken dishwasher. It’s an absurd scene, but they’re just blotting out the carnage in the same way I am – by going through the motions.

Back on street level, a passing firefighter asks to use my Nokia to call his family. He can’t get through so dashes off towards Ground Zero. The number lingers on my phone for weeks after. I’ll often think of calling it to see if he came back alive, but I’ll never work up the nerve.

I’m assigned to the hospital watch for the rest of the day, told to photograph the injured as they are brought in. It’s quiet at St Vincent’s so I trek over to the Chelsea Piers, where a vast entertainment complex has become an emergency triage centre.

It’s one of the most chilling scenes of my day. Ambulances and medics have raced here from far-flung places like Rhode Island to help, but they’re all standing idle. Rows of beds are empty. It slowly becomes apparent that no one caught up in the collapse got out alive.

Later in the night, I walk home across Manhattan. Around the world my photo is rolling off the presses and being circulated online. Colleagues are praising me for standing firm when others around me were running. I feel numb, the uncontrollable sobbing still days away.

For years afterwards I thought that photo had nothing of me in it – I’d simply held a mirror up to other people caught in a horrific event. But that’s not true. My life changed quickly after I’d taken it. Career doors opened and I landed assignments in Afghanistan, Indonesia and around the world before settling in the UK with an English husband I met on my travels.

I’ve been in touch with two of the people in the photo and to an extent we’ve bonded over that shared moment. Today I look at that image and see myself as I was 15 years ago. A young photographer, turning towards a scene of terrible destruction. Snatching 1/200th of a second of clarity from the chaos to come.