On the morning of Sept. 11, 2001, Thuc Doan Nguyen-Brophy was startled awake by a violent crash. The sound was so loud and seemed so close, she said, that she thought a delivery truck had slammed into her apartment building on the corner of Canal and Essex Streets in Lower Manhattan.

Ms. Nguyen-Brophy had moved to the city after college three years earlier with a boyfriend. During the day, she temped in corporate offices. At night, she worked as a go-go dancer in gay bars. She felt like she had finally found her tribe downtown — below 14th Street was her self-imposed boundary.

Her family had come to America as refugees from Vietnam in 1980, and she had faced discrimination growing up. She found a sense of safety and belonging in New York’s nightlife scene among drag queens. She bought a mini digital video recorder to film her clubland adventures and conversations with friends.

Ms. Nguyen-Brophy got up from the sofa where she’d been sleeping. From the window of her second-floor apartment, she saw a crowd beginning to gather on the street below. They were facing west, toward the World Trade Center. The north tower was engulfed in thick black smoke.

Instinctively, Ms. Nguyen-Brophy grabbed her JVC video camera, climbed out onto her fire escape and began filming. She zoomed in and pulled back from the Twin Towers, adjusting the focus and angle, panning from the burning building to the spectators on the sidewalk below her. In total, she kept the camera going for seven minutes and four seconds — long enough to capture United Airlines Flight 175 hitting the south tower.

Ms. Nguyen-Brophy left the city three months after the attack. The office work and club gigs dried up in the wake of the tragedy, and seeing the faces of the missing or presumed dead on posters around town overwhelmed her.

She moved to Los Angeles, where at first she recreated her life in New York, temping for entertainment companies and dancing in gay bars. She lived in California until the coronavirus pandemic.

Ms. Nguyen-Brophy said she did not look at the footage for more than two decades. She rarely spoke about the tape, which she kept in a small safe along with her passport and other papers. She feared it contained images of men and women who jumped from the burning buildings to their death.

Then, in 2022, while unpacking after another move (this time to New Orleans), Ms. Nguyen-Brophy resolved to do something with the footage. She’d read that Minidisc tape can degrade with age, and she was tired of dragging it around from place to place.

Last year, Ms. Nguyen-Brophy donated the physical tape and the license to show the footage to the 9/11 Memorial and Museum. The museum actively collects and solicits donations of photographs and video related to the Sept. 11 attacks. Footage is examined to corroborate the person’s motivations for chronicling what they witnessed or experienced. A metadata analysis of Ms. Nguyen-Brophy’s video confirmed it was taken on Sept. 11, 2001.

Dylan Williams, associate curator of the 9/11 Museum, said the amateur clip is a visual artifact from an age before everyone carried a high-resolution camera in their pocket. “With social media, things are scripted before they ever get filmed and posted,” Mr. Williams said. “So footage like Thuc’s is remarkable because it really does animate a moment in time.”

For Ms. Nguyen-Brophy, now 50, the act of donating the tape was also a reckoning with long-buried feelings and her former life in New York.

She has returned to the city to grapple with what she witnessed and recorded. And she knows the question everyone will ask.

“I purposely didn’t watch it because I knew it would make me feel badly,” Ms. Nguyen-Brophy said, sitting at a cafe on Essex Street in Chinatown, near her old apartment. “I never bring it up in therapy. None of my friends ever brought it up because no one knew I was there.”

Ms. Nguyen-Brophy was accompanied by Keith Maitland, a filmmaker, and Alisa Payne, a producer. The two are working on a short documentary titled “My Fire Escape” about Ms. Nguyen-Brophy’s footage and experience of Sept. 11, as well as the larger experience of the residents of Chinatown, which they say remains an untold story.

Ms. Nguyen-Brophy said she has a tendency to dissociate in moments of fear and danger. One of the things about Sept. 11 she half-buried, she said, was that had she not missed a call that morning from her temp agency, she might have been working the reception desk at Oppenheimer Funds, a firm that occupied several floors of the south tower.

Now a writer, Ms. Nguyen-Brophy has visited New York many times in the last 25 years. But until this summer, she had not returned to her old block.

“I actually avoided corners,” she said. “I’d go down Essex, feel funny and leave.”

Ms. Nguyen-Brophy had lost the digital camera. And Minidisc technology is antiquated. After the museum digitized her tape, she was able and finally ready to watch the footage she’d been carrying around so long. She was surprised at how calm she sounds as she confers with the onlookers below trying to make sense of an event that defies comprehension.

At first, Ms. Nguyen-Brophy said, she thought the smoke from the north tower might be Hollywood special effects for a movie shoot, something out of “Die Hard.”

“It was an airplane,” a man in a white T-shirt says, looking up at Ms. Nguyen-Brophy and her camera.

“An airplane?” another man asks in disbelief.

“An airplane crashed into it,” the first man repeats. “I mean, that could have been a terrorist act.”

Around the halfway mark of Ms. Nguyen-Brophy’s video, Flight 175 looms into view in the clear blue sky behind the towers. Ms. Nguyen-Brophy’s footage goes black for a few seconds. She doesn’t know why. Her best guess is that she stopped recording briefly. The clip resumes just as the second plane hits the south tower and explodes.

Mr. Williams, the museum curator, said much footage exists of the second plane strike. What makes Ms. Nguyen-Brophy’s tape significant is its unscripted and unknowing quality.

“It does convey the shock, the confusion, the horror in real time,” he said. “It’s raw. Yet it’s taken from the intimate confines of someone’s apartment.”

The footage is available upon request to researchers. And it will feature prominently in the forthcoming documentary. Ms. Nguyen-Brophy’s tape and others like it may also soon be viewable by the public through the museum’s website, Mr. Williams said.

When Ms. Nguyen-Brophy visited her old apartment this week with the filmmakers, she cried, unexpectedly. “That was my final home here,” she said. “I never thought I would stay in Los Angeles. I thought I would move back.”

Going though her things recently, Ms. Nguyen-Brophy found another image from that period — a photograph taken six weeks after the attack, on Halloween night. She and a friend are standing on the sidewalk in the East Village in their costumes, “trying to be regular 25-year-olds like in the before times,” she said.

On a wall behind them is a poster of an American flag and the motto during those grim days: United We Stand.