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.

The flag’s return required a transcontinental trip; it was recovered in Washington state.

On Sept. 11, 2001, three firefighters — Billy Eisengrein, George Johnson and Dan McWilliams — removed the flag and its pole from the Star of America, a yacht that had been moored at the North Cove marina on the Hudson River. Thomas E. Franklin, a photographer for The Record, a newspaper in northern New Jersey, captured the moment when it was raised at ground zero.

After flying at ground zero, the flag, or so it was believed at the time, was unfurled at Yankee Stadium and on naval ships in the Middle East before returning to City Hall in 2002. But it turned out the flag — which was signed by Gov. George Pataki and Mayors Rudy Giuliani and Michael Bloomberg, among others — was not the right one. The yacht’s flag measured 3 feet by 5 feet; the autographed flag was 5 feet by 8 feet.

The flag’s whereabouts was not known until late 2014 after the mystery was featured on the first episode of “Brad Meltzer’s Lost History,” which was broadcast on H2, a spinoff of the History Channel.

“All we did was put up the modern-day version of the wanted poster,” said Meltzer, an author of thriller novels and children’s books as well as a writer for television. “We put that flag on TV.”

The episode, which uses a grainy video to show that the flag disappeared on the night of the terrorist attack, was broadcast on Oct. 31. Four days later, a man carrying a plastic bag walked into a fire station in Everett, Washington. He had seen the show and believed he had the flag.

“He used the name Brian,” Mark St. Clair, the deputy chief of operations at the Everett Police Department, said. “At least that’s what the firefighters recall him using.”

Brian said he was a former Marine who had been deployed in the Middle East. He was given the flag, according to the firefighters’ account provided to St. Clair, by a worker at the National Oceanic and Atmospheric Administration, who had in turn been given the flag by the widow of someone killed on Sept. 11 in New York.

St. Clair said it was unclear whether the firefighters knew about the 9/11 flag or that it was missing. But out of respect for the flag that was turned in, they unclipped it from its halyard, folded it and told their superiors. The police began investigating.

“In the beginning it was just a flag that was returned,” said Kim Gilmore, a senior historian at the History Channel. “There was a lot of work to determine if it was the ground zero flag.”

St. Clair gathered footage from cameras near the firehouse and released police sketches of Brian, based on descriptions from firefighters, to a local newspaper, but that effort did not yield results. The sketch also did not provide any leads at NOAA.

On Sunday, the 15th anniversary of the attacks, the History Channel will broadcast a program on the flag’s recovery. During the show, John W. Cutter, who was a member of the New York Police Department’s criminal intelligence section and retired as a deputy chief in 2004, expresses doubts that the flag came from a widow. Flags used at burials typically do not have any rope or hardware. “This leads me to believe that he received the flag in some other fashion and is afraid to say how he got it,” Cutter said, referring to the Marine.

Another figure central to the effort to verify the flag’s authenticity was Bill Schneck, a forensic materials scientist with Washington state Patrol Crime Laboratory who had previously worked for the McCrone Group, a testing firm in Westmont, Illinois, that specializes in particle identification.

“In this case, I compared the dust on the flag to known dust samples collected by others in the days after 9/11,” Schneck said. That dust was a mixture of concrete, glass fibers, plastic, molten pieces of metal and asbestos. “It was like what you would call a fingerprint,” Schneck said. His analysis of the particles on the flag showed the same characteristics as particle types in the dust after the towers fell.

Schneck also conducted an extensive photographic comparison of the flag in Washington to high-resolution images of the Sept. 11 flag taken by Franklin and provided by The Record. The two flags were the same size and made from the same material, and each had a brass and a stainless steel snap and halyards with distinctive knots and wraps of black electrical tape.

The History Channel enlisted Monica Rosero, who worked on the Star of America, to help with the identification. She recognized the hardware and rope and said the black tape was the handiwork of her husband Carlos Rosero, the boat’s engineer, who died in 2008.

By July, the findings of the investigation “led us to believe that we had enough information that we could release that flag to someone claiming ownership,” St. Clair said. The flag was then sent back to New York. How the flag ended up across the country is a piece of the puzzle that remains unsolved.

Among those who were informed that the flag had been found was Shirley B. Dreifus who, along with her husband, Spiros E. Kopelakis, owned the Star of America.

“It’s truly amazing,” Dreifus, who sold the yacht in 2008, said. “In fact, ‘stunning’ is the way I put it.”

Dreifus said she was saddened that her husband, who died nearly two years ago, could not share in the news. “He called it the icon of the century,” she said. “This was the only symbol of hope that day.” She and the Chubb insurance company, which paid a claim on the flag, donated it to the Sept. 11 museum.

Joseph C. Daniels, the chief executive and president of the memorial and museum, said: “To not have that flag as a part of the museum, it always felt like there was something missing. It was a symbol of not only hope, but of strength. We needed both at the time.”