Usually, she said, the symptoms are brought out by “a trigger in the here and now.”

The triggers could include ordinary elements of daily life. For example, a bright late-summer morning could serve as a reminder of Sept. 11, 2001, and touch off memories of the planes taking aim at the twin towers. Sounds could have a similar effect.

“From the time I was 17 on, I couldn’t listen to an airplane overhead or a helicopter, or a car backfiring,” said Lila Nordstrom, who was a senior at Stuyvesant High School in Lower Manhattan in 2001 and now lives in Los Angeles. “If I was in traffic and someone honked, it would lead to a protracted period of me panicking. It took a while for me to realize it was happening, and it took a while after that to realize it was 9/11-related.”

She said that she was certified for post-traumatic stress disorder under the World Trade Center Health Program in 2015.

For Elinda Kiss, a business school professor who was attending a conference in the Marriott World Trade Center hotel on Sept. 11, driving past Newark Liberty International Airport on the way to teach at the Rutgers University campus in Newark would bring a replay of her frantic run to safety.

“I would see the planes coming in for a slow landing,” she said. “I would relive seeing the plane hit” the south tower.

Dr. Kiss, who now teaches in the Robert H. Smith School of Business at the University of Maryland, said she assumed her post-traumatic stress had largely disappeared over the years. But on Sunday night, lying awake, the nightmare replayed again and again. “I kept seeing the plane hit in the middle of World Trade 2,” she said.

Ms. Bergeron, the former Port Authority official, said she took almost no time off after the attacks. “There was no time to grieve,” she said.