On the morning of Sept. 11, 2001, the 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.

Mr. 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.

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