Firefighters raise an American flag at the site of the World Trade Center. (Thomas E. Franklin/Bergen County, N.J., Record via Getty Images)

Shortly after three New York firefighters raised an American flag above the ruins left by the Sept. 11 attacks — a moment captured in an iconic photo seen around the world — the flag disappeared.

Now, 15 years later, the treasured Ground Zero artifact has been recovered — nearly 3,000 miles away in Everett, Wash.

Where it ended up remained a mystery until late 2014, when a man who introduced himself as Brian went to a fire station in Everett carrying a flag in a plastic bag, according to Q13 Fox, a local affiliate.

The man said he had seen a documentary about the disappearance and believed he had the missing flag.

Bill Schneck, a forensic materials scientist for the Washington State Patrol Crime Lab, was soon tasked to determine its authenticity.

“In this case, I compared the dust on the flag to known dust samples collected by others in the days after 9/11,” Schneck told the New York Times. “It was like what you would call a fingerprint.”

Schneck’s analysis showed that the dust samples from the flag have the same characteristics as the ones found at Ground Zero.

Schneck also compared the flag with high-resolution images captured 15 years ago by newspaper photographer Thomas E. Franklin, who was at the World Trade Center site on Sept. 11 when the flag was raised, the Times reported.

The analyst deduced that the two flags were the same size, were made from the same material and had other similar qualities, according to the Times.

“If you look at the original photograph from 9/11 you see a certain electric tape holding two lines together, and they had the same general profile,” Schneck told the local Fox affiliate. “So I knew then, well, this could be it.”

It is believed that the flag was hanging on the pole of a yacht moored at a small marina just west of Lower Manhattan on the morning of the World Trade Center attacks.

Hours later, firefighters George Johnson, Billy Eisengrein and Dan McWilliams removed it, then raised it above the rubble.

Franklin, a photographer for the Record newspaper in Bergen County, N.J., captured an image of the firefighters as they raised the flag — though he didn’t know at the time that he’d made an iconic photo.

“I was like, ‘I need to go back,’ and that’s when I saw the three firemen; they were fumbling with the flag,” Franklin told Politico in 2011. “It was shot at a distance, and it just happened.”

He added: “This picture did not stand out to me. Three men raising a flag paled in comparison to thousands of people dying and two buildings falling to the ground. I can’t even say this is the best picture I ever took. It is the photo with the most meaning.”

The photograph was moved to the Associated Press wire that night and wound up on front pages and magazine covers around the world. There were comparisons, too, to Joe Rosenthal’s 1945 picture of six men raising an American flag atop Mount Suribachi on Iwo Jima — one of the most enduring images of World War II.

“The picture and the level of attention it got, for a while was unprecedented,” Franklin told Politico. “I still don’t lose focus on the fact that thousands of innocent people died. That was a murder scene, and people died in the most horrific way.”

Eventually, a flag believed to be the Ground Zero artifact from Franklin’s photo toured the world, showing up at benefit concerts and aboard military ships en route to war.

It was autographed by George Pataki, New York’s governor at the time of the terrorist attacks, and two New York City mayors, Rudolph Giuliani and Michael Bloomberg.

But it turned out it wasn’t the right one; the touring flag was larger than the Ground Zero flag, which apparently vanished just hours after Franklin photographed it.

How it disappeared remains a mystery.

According to the Times, “Brian” introduced himself as a Marine veteran who had been deployed to the Middle East. A widow of a 9/11 victim is said to have given the flag to a worker at the National Oceanic and Atmospheric Administration, who in turn gave the flag to “Brian,” the Times reported.

Everett police have so far been unsuccessful in trying to find the mysterious man who walked into the fire station in 2014, but a composite sketch of the man based on descriptions from firefighters has been released in the hopes that somebody can identify him.

Now recovered, the original Old Glory has been unveiled at the National September 11 Memorial Museum, where it will remain on display.

“In the darkest hours of 9/11 when our country was at risk of losing all hope, the raising of this American flag by our first responders helped reaffirm that the nation would endure, would recover and rebuild, that we would always remember and honor all of those who lost their lives and risked their own to save others,” 9/11 Memorial President Joe Daniels said in a statement. “We had always hoped this special flag and its story would be shared with our millions of annual visitors coming from around the world, and for that, we are thankful.”

The History Channel will air a documentary about the flag’s recovery on Sunday, the 15th anniversary of the attacks.

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