Amid all the iconic photos of September 11, it’s easy to forget those on the periphery of the event—the people who came together in shock and sadness to watch and process what they were seeing. Photographer Robert Clark took some of the most widely published photos that day, including a series of four images he shot from his rooftop in Brooklyn that show the slow-motion collision of the second plane hitting the towers.

Now, on the 15th anniversary of the attack, Clark has shared with National Geographic a never before seen image from 9/11, one that shows the tender side of a day of horror as his neighbors gathered to watch the smoke rise, drawn together by a collective sense of