Almost immediately after the attack on the World Trade Center on Sept. 11, 2001, dazed New Yorkers came together to mourn, and to start making sense of what happened. For some, that meant creating shrines in parks and in front of firehouses. For the filmmakers Steven Rosenbaum and Pamela Yoder, it meant picking up their cameras — but also, taking out a six-line classified ad in The Village Voice.

“Do you have video footage from the week of 9/11?” the ad asked. “You can contribute to history.”

“Wherever you were, whatever you saw,” it continued, you can help build “a testament to our city’s heroism, pain, strength and resilience.”

More than 100 people responded with footage shot from apartment windows and rooftops, on street corners and in parks. The material captured both the devastation of the attack and the collective mood of the immediate aftermath, when children chalked drawings of flowers next to mangled cars, and strangers gathered to process and, sometimes, argue.

Some of the footage was used in the couple’s 2002 documentary, “Seven Days in September.” But since then, the more than 500 hours of footage has gone largely unseen.

Now, the New York Public Library has acquired the couple’s archive, which it says is the largest collection of video documentation of the attack and the days just after. It is being donated along with more than 700 hours of behind-the-scenes footage the couple also captured of the long, contentious process of creating the 9/11 Memorial & Museum in Lower Manhattan.