At the time, he says, his thought process was simply, "This country got attacked, there's all this devastation, thousands of people died; let's do something good right now."
The whole thing took no more than five minutes, just long enough to be captured by The Record photographer Franklin.
After taking pictures all day, Franklin said, he had nearly run out of space on his digital camera when he saw the firefighters hoisting the flag.
"I was about 30 or 40 yards away," he said. "Then all of a sudden the flag starts to go up this pole. I shot a burst of frames. It was over like that."
Initially, The Record ran the photo without identifying the firemen. Franklin hadn't tried to interview them afterward.
"It was a serious, serious news situation," he said. "I didn't talk to them, they didn't talk to me. We walked right past each other."
But after releasing the photo to the Associated Press the morning of Sept. 12, 2001, the picture started getting more attention. The Record decided to find the firefighters, who said they were unaware they had been photographed.
"To this day I still receive phone calls, emails and letters from people telling me what the picture means to them," Franklin said, adding he had gotten a letter in August from a young boy in the Midwest who asked for his autograph. "For whatever reason, people connect with this picture, even 10 years later."
The picture has been featured on postage stamps, posters and even Christmas ornaments (Eisengrein's mother puts one on her tree every year). The image has also been molded into a 40-foot-high, bronze-and-steel sculpture unveiled in 2007 at the National Emergency Training Center, north of Washington, D.C.
As for the original photo, a Queens pharmaceutical executive bought the picture, signed by Franklin, in 2002. Stewart Rahr, president of KinRay, paid $89,625 for the image at a Christie's auction and hung the photograph in the lobby of his company's headquarters.
"There were so many amazing photographs made that day. It was probably the most photographed event in history," Franklin said. "So many [of them] are of the horrific nature of buildings, planes crashing into buildings -- thousands of people running for their lives. This is a picture that's really about the opposite, which is hope and strength, and solidarity."
The Firemen's Code
In the years after 9/11, Eisengrein and the other firefighters in the picture have declined nearly all interview requests, preferring to remain private.
Their decision stems, in part, from the photo's having been compared to the flag-raising at Iwo Jima, Eisengrein said, noting pointedly that most people in America can't name the men in that picture.
"We're pretty adamant about not letting that change who we are and what we are," he said. "Let the picture stand for itself."
Sean Nealon, 47, a former member of the Navy who has been with Rescue 2 for seven years, says it's still difficult to talk about 9/11, especially to those who weren't there.
"We talk about it amongst ourselves," he said. "I really can't describe the whole scene then, even 10 years removed."
Every year, on the 9/11 anniversary, a priest visits Rescue 2 and they hold a Catholic mass in the firehouse. They're planning to do the same this year.
At the firehouse, the men clam up at questions, wary of revealing too much about each other, or themselves. To do so would be frowned upon: You do not brag, or bask in your accomplishments. When asked about Eisengrein, after the guys exchanged a few jokes at Eisengrein's expense, Nealon would only say, finally, "He's a nice guy."
"These guys didn't do this because of the picture," he added. "I don't think anyone really feels what they did is deserving of the recognition. It's a team sport."
Firefighters' Charity: The Bravest Fund
Neither Nealon nor Eisengrein have suffered any 9/11-related health problems, but thousands of others did after inhaling the dust and debris. So during the fall of 2001, the three firefighters used money earned from licensing their photo to start a charity called "The Bravest Fund."
They're open to helping "pretty much anybody," Eisengrein said. They started out wanting to help emergency workers who operated at the World Trade Center, then broadened their mission, advertising by word of mouth. In some cases people were seeking help with unpaid medical bills.
"We've helped a lot of firefighters' families, a number of construction workers, and a few other people ... that knew we had this," Eisengrein said. "If there was money to be made, why not put it where we wanted?"
As of this year, Eisengrein said, they have given away more than $1 million dollars to people who have illnesses that aren't covered by insurance.
"There are only a few thousand dollars left," he said.
In 2004, then-Attorney General Eliot Spitzer's office examined the charity's use of its funds, but took no subsequent legal action.
Life After 9/11
Today, Eisengrein is still living in Staten Island, the same borough where he grew up. During his time off he practices carpentry, and recently finished siding a house.
But most of all, "I ride my motorcycle as much as possible," said Eisengrein, who has been a motorcycle instructor for the past six years and owns two Harleys. On a recent weekend he went to Ocean City, Md., with a few of the guys from Islanders MC, a Staten Island motorcycle club.
He hasn't gotten much attention lately, he said, but "every once in a while I get a letter from someone," often requesting an autograph.
When he looks back at 9/11, Eisengrein said, he doesn't think about the heroic gestures, he thinks about the lives lost -- all 2,973.
But he, McWilliams and Johnson are honored people have found the photo of their flag-raising reassuring. In the past 10 years, people have told Eisengrein time and again that the image helped them understand, "we as a country, were going to be OK."
"The three of us are very proud ... and honored to have been a part of that," he said.