NEW YORK, NY.- On the morning of Sept. 11, 2001, photographer Thomas Hoepker was following the instincts of a lifetime of documenting the human condition: trying to get close to his subject.

With subway lines out of service, he jumped in his car on the Upper East Side, crossed the Queensboro Bridge and sought an alternate route to the southern tip of Manhattan.

In Williamsburg, Brooklyn, he saw out of the corner of his eye an arresting scene. A group of five people were lounging on a gentrified stretch of waterfront, focused on one another while seemingly unperturbed by the horrific plume of smoke marring a late summer day as the World Trade Center towers burned.

Hoepker shot three quick frames and got back in his car.

The picture — which he withheld from the public for five years because, he said, it didn’t “feel right” — became one of the indelible images of 9/11, mesmerizing viewers, provoking controversy and raising questions about the ambiguity of a photograph.

Hoepker, a German-born photojournalist with the Magnum Photos agency, died Wednesday in Santiago, Chile. He was 88.

His death was announced by Magnum, which said he had Alzheimer’s disease.

Hoepker’s career spanned decades of a golden era for magazine feature photography, beginning in the 1960s, when he won assignments to shoot a long road trip across America and to document Muhammad Ali as he trained in London and Chicago in 1966.

He was on the staff of the German weekly newsmagazine Stern for many years beginning in 1964, and he was director of photography for the American edition of the travel and exploration magazine Geo from 1978 to 1981. He published books on Ali, the Maya of Guatemala, life in East Germany and many other subjects.

But the image that came to most define his career was the one he took on the morning of 9/11. Its seemingly idyllic mood juxtaposed with tragedy has drawn comparisons to “Landscape With the Fall of Icarus,” a Renaissance painting, attributed to Bruegel, that depicts farmers in their fields whose backs are turned indifferently to a boy who has just dropped out of the sky and appears to be flailing in the water.

The first time that Hoepker’s photo was published was five years after the terrorist attacks, in a book by David Friend, “Watching the World Change: The Stories Behind the Images of 9/11.”

Hoepker told Friend that he hadn’t published the picture earlier because the actions of the people in it troubled him. “They were totally relaxed like any normal afternoon,” he said. “It’s possible they lost people and cared, but they were not stirred by it.”

New York Times columnist Frank Rich, writing on the fifth anniversary of the attacks, found Hoepker’s photograph to be a metaphor for the country’s failure to absorb the lessons of that day.

“What he caught was this: Traumatic as the attack on America was, 9/11 would recede quickly for many,” Rich wrote. “This is a country that likes to move on, and fast.”

But others rejected the judgment that the five unidentified people were behaving callously. A man named Walter Sipser wrote to Slate magazine saying that he was one of the people in the picture, and that he and his friends had not been indifferent.

“We were in a profound state of shock and disbelief, like everyone else we encountered that day,” Sipser wrote. “Had Hoepker walked 50 feet over to introduce himself he would have discovered a bunch of New Yorkers in the middle of an animated discussion about what had just happened.”

The true meaning of the photograph, he added, was how easy it was to manipulate and misconstrue an image.

Five years later, in 2011, Jonathan Jones, an art critic for The Guardian, wrote that the feelings of the people in the photograph and the photographer’s intentions were irrelevant to the cultural meaning the image had acquired — an allegory of history and memory.

“It is the only photograph of that day,” he wrote, “to assert the art of the photographer: Among hundreds of devastating pictures, by amateurs as well as professionals, that horrify and transfix us because they record the details of a crime that outstripped imagination — even Osama bin Laden dared not expect such a result — this one stands out as a more ironic, distanced, and therefore artful, image.”

Hoepker defended his photograph because of its ambiguity.

“I think the image has touched many people exactly because it remains fuzzy and ambiguous in all its sun-drenched sharpness,” he wrote in Slate in 2006. “On that day five years ago, sheer horror came to New York, bright and colorful like a Hitchcock movie.”

Thomas Hoepker was born in Munich in 1936. He began taking pictures at 14 with a simple glass-plate camera that was a gift from a grandfather. As a young man, he worked for German publications Münchner Illustrierte and Kristall.

One of his early assignments for Kristall was to cross the United States, on a journey inspired by photographer Robert Frank’s book “The Americans.”

His photographs of Ali in training in the mid-1960s produced two of his best-known images: the heavyweight champion jumping playfully on a bridge over the Chicago River, and an extreme close-up of Ali’s right fist as he throws a punch.

For Stern, Hoepker worked with his second wife, Eva Windmoeller, a journalist, first in East Germany and then in New York, where they moved in 1976. New York became his home for almost five decades.

Magnum Photos, a photographer-owned cooperative founded by Robert Capa, Henri Cartier-Bresson and others, began representing his archives in 1964. He became a full member in 1989 and served as its president from 2003 to 2006.

When he was diagnosed with Alzheimer’s in 2020, Hoepker and his third wife, Christine Kruchen, retraced parts of his mid-1960s road trip across the United States. His contemporary color photographs were published alongside his earlier black-and-white images in a book, “The Way It Was,” in 2022. The 2020 trip was also the basic of a 2022 documentary film, “Dear Memories.”

Kruchen survives him. Complete information about survivors was not immediately available.

“From the beginning, I always had an interest in people, not so much in buildings or still life,’’ Hoepker told The Business Times in 2018. “I was a street walker and took pictures of whatever I found interesting.”

This article originally appeared in The New York Times.