The moment was captured in a photograph, an unforgettable image that was seared in the world’s collective memory as a symbol of resilience: three firefighters raising an American flag amid the ruins of the World Trade Center on Sept. 11, 2001.

Soon after, the flag disappeared. New York City officials tried to track it down, to no avail.

Now the flag has been recovered, though the mystery has not been entirely solved. On Thursday, nearly 15 years after it first flew, the flag will again become a prominent part of Lower Manhattan when it goes on display at the National September 11 Memorial & Museum.