Photographer Jenny Warburg knows what a terrorist attack looks like. In 1995, she went to Oklahoma City to take pictures of the site where anti-government activists blew up a federal office building, killing 168 people.

A political activist and former social worker, Warburg also knows what extremism looks like. Both as a contributing photographer for The Independent and as a freelancer for numerous national and international publications, she’s spent more than a decade documenting activities of ultra-conservative religious groups, violent extremists and neo-Nazi organizations.

But nothing could really prepare Warburg for her latest photographic encounter. She was on a shoot in Canada on Sept. 11 when terrorists flew two jet planes into the World Trade Center in New York City, killing thousands and momentarily stopping the clock of world history. After photographing thousands of the stranded airline passengers rerouted to Canada, Warburg rented a car and sped south across the border to her hometown. When she got there, she secured the necessary police pass and headed for “ground zero” in lower Manhattan.

What struck her most about what she saw in her subsequent 10 days in New York was not the chaos, but the ghostliness of much of the city, especially below Canal Street.

“What’s eerie for anyone who knows New York is how empty and quiet it was,” Warburg says. “Except for the patrolling fighter pilots and helicopters, the sky was empty. The streets were empty. It was very weird, for example, to be able to see the whole façade of the New York Stock Exchange building. Wall Street is usually crowded with people.”

A frightening contrast to the emptiness was the martial clutter of police lines, National Guard troops, U.S. Army vehicles and Coast Guard boats that were everywhere in view. Also everywhere were the homemade shrines, posters describing missing relatives, emergency vehicles and piles of donated food, clothing and equipment.

Some of what she saw, Warburg chose not to photograph. “I didn’t want to do the usual and get directly in people’s faces,” she says. “I saw their expressions when they looked at the skyline; when they looked at what was missing or when they read the posters of missing persons.”

Other images that will stay with her: the buckets full of body parts being passed along a line of rescue workers; a pair of severed hands–one black and one white–still clasped together on a pile of rubble.

Amid an overwhelming atmosphere of helplessness, for Warburg, it was the “little human moments” that brought the impact of what’s happened to New York most clearly into focus.

“It would catch you off guard when someone would say something ordinary to you and suddenly you’d be crying,” she says. “The image that keeps coming back to me is that empty sky and then the fighter pilots in the air.”

How do you photograph something that’s absent? How do you document unimaginable loss? Here’s how Warburg does it.