"We stood there and looked at it for a second and went about our ways," William "Billy" Eisengrein told ABCNews.com.
Eisengrein and the two other firefighters had no idea they were being photographed, but the picture of them hoisting the flag above the rubble quickly became well-known around the world.
Now, after years of silence, Eisengrein spoke about 9/11, and the "moment of time with three guys" that still remains a symbol of America's strength and resilience.
"From the moment the picture was published, it has lived a life of its own," said Thomas E. Franklin, who took the picture of the firemen for The Record, a daily New Jersey newspaper.
Today, Eisengrein — pictured on the far right — is 47 years old and still a firefighter. His arms are covered in tattoos: on his right, a clearly visible image of the Twin Towers, inked in 2002.
"It just seemed like the thing to do at the time," he said.
The FDNY veteran, now in his 26th year, clearly remembers the morning of 9/11. It was bright and sunny in New York City, and the sky was blue. Six men on their shift at Brooklyn Rescue Company 2, where Eisengrein has worked for 17 years, arrived at ground zero first and went to the North Tower. They all died, along with many more.
"I lost 100 friends that day," Eisengrein said.
He heard the news about the first plane hitting the World Trade Center while he was at his girlfriend's home, watching TV. And then the second plane came. That's when Eisengrein hopped on his motorcycle and headed to the Brooklyn firehouse, reasoning his bike would be the best way to navigate traffic.
"I saw the plane crash into the tower, and I said, 'Alright, I have to go to work,'" he said.
When he arrived at ground zero, at about 10:45 a.m., one of the most eerie parts, he said, was the "absolute silence" accompanying the dust and papers strewn throughout the downtown area. Around 5 p.m., after spending the entire day searching the pile, all emergency responders were told to leave, out of fear building 7 was going to come down.
Eisengrein was sitting on the front bumper of his rig when he noticed two other firefighters carrying a flag. One of them was his good friend, Daniel "Danny" McWilliams, now an FDNY lieutenant in Brooklyn in his 21st year with the department. The other person was someone Eisengrein didn't know at the time, but would soon be linked with for the rest of his life: George Johnson, who is now in his 20th year with the FDNY and a battalion chief in Brooklyn.
Both McWilliams and Johnson declined interviews with ABCNews.com.
As the story goes, McWilliams, who had grown up with Eisengrein on Staten Island, found the flag on a 130-foot yacht in the Hudson named "Star of America," owned by Shirley Dreifus of the Majestic Star Co. in New York. As soon as Eisengrein saw McWilliams, he said, "I knew he was going to put the flag somewhere."
He hollered out, "Do you need help?" then joined them in looking for a place to hang it.
A couple minutes later, they discovered a construction trailer on the northwest corner of Liberty and West, with a big flagpole leaning against it.
"So we put a piece of tin on the ground up to the trailer and hiked up that, and raised it," Eisengrein said.
Time for a CGar for the Heros!