Shortly after the twin towers fell 18 years ago, a photo of three firefighters raising an American flag in the rubble went viral online.

That flag eventually went missing, and stayed missing for several years, until author and History Channel Host Brad Meltzer managed to track it down through his television show, "Lost History."

Meltzer said it took years to track down the flag, which, he added, is greater than the sum of its parts.

"That flag is not an object. It reveals us. It symbolizes who we are," he said. "That flag is not a red, white and blue piece of cloth. It is absolute proof that this country, when we are kicked in the teeth, we will get back up again."

The flag is now enshrined at the 9/11 Memorial in Manhattan.

Nearly 3,000 people were killed on September 11, 2001, in the worst terrorist attack on US soil.

WGBH's Henry Santoro sat down with Meltzer in our WGBH studios to learn how Meltzer relocated the missing flag.