Debi Grossman, who remembers fighter jets rocketing over the Washington region’s skies when airliners struck the World Trade Center in New York and then the Pentagon, wanted to mark the 15th anniversary of the terrorist attacks by volunteering in any way that she could.

Her husband, Bill Bachman, said he wanted to honor the memory of the day by giving back to his community, “to pay it forward.”

Falak Z. Ghatala — who only recently began wearing a headscarf as a more public display of her Muslim faith — joined Grossman, Bachman and thousands of others on the national Mall to mark the day by donning red aprons and packing meals for elderly Americans as part of a national day of service.

In interviews, these Washington residents and many others spent part of Sunday reflecting on the day 15 years ago that pitched the United States into a stubborn and ruthless war in Afghanistan, and became the prelude to a broader conflict that has touched almost an entire generation and still seems to have no end in sight.

“Panic and tears — that’s all I remember,” Ghatala, 35, said. Then a student at the Stevens Institute of Technology in Hoboken, N.J., Ghatala watched the twin towers burn from across the Hudson River. “It was very, very scary for us for a while.”

Ghatala, who is national program director for a New Jersey-based nonprofit organization that fights hunger, said part of the reason she began wearing a hijab was because she felt that Muslims have come under unfair scrutiny. So she wanted to show her pride in her religious identity as she also worked to make the world a better place by doing things such as boxing meals for others.

“I think it makes me realize how important it is to stand up for the right,” Ghatala said.

The event — which was held in a large tent near the National World War II Memorial and sponsored by the AARP Foundation and the Capital Area Food Bank, among others — drew more than 2,000 volunteers by midday.

Lisa Marsh Ryerson, president of the AARP Foundation, said approximately 1 million meals were boxed Sunday, and a half-million were expected to be packaged Monday. An estimated 5,000 volunteers were expected to take part in the Million Meal Pack Challenge, she said.

“I think, nationally, what we’ve done to come together after 9/11 — to really, on a solemn occasion, pay tribute and honor by coming together in service to others — is really important,” she said.

The event was one of several held in the region — and dozens nationwide — as the nation marked the anniversary. Mourners gathered at the three sites hit by terrorists; nearly 3,000 people died in the attacks.

In New York, thousands of people gathered in Lower Manhattan for a ceremony at Ground Zero that was timed to coincide with the moment the first of the jetliners struck on Sept. 11, 2001. Organizers estimated that 8,000 people listened to a roll call of the dead. A new tower has since risen over the New York skyline where the Twin Towers once stood, but memories of that day — and the pain of loss — were still fresh.

“It doesn’t get easier. The grief never goes away. You don’t move forward — it always stays with you,” Tom Acquaviva, who lost his son, Paul, told the Associated Press.

For Dorothy Esposito, the passage of 15 years feels “like 15 seconds.” Her son, Frankie, was killed.

James Johnson was at Ground Zero for the first time since he last worked on the rescue and recovery efforts in early 2002, when he was a New York police officer. The 9/11 museum and memorial plaza, three skyscrapers and a transit hub have been built on land that was a disaster zone when he last saw it.

“I’ve got mixed emotions, but I’m still kind of numb,” said Johnson, now a police chief in Forest City, Pa. “I think everyone needs closure, and this is my time to have closure.”

New York Mayor Bill De Blasio (D) attended, along with presidential candidates Hillary Clinton and Donald Trump. Clinton, who in 2001 was a U.S. senator representing New York, fell ill during the service and left early.

Much has changed in the 15 years since the attacks. One World Trade Center now glistens above a revitalized Lower Manhattan. On most weekdays, thousands of New Yorkers move through the streets below, but in the early hours Sunday, the narrow roads were sparsely occupied.

The family of Erwin Erker, who perished in the North Tower, was among many quietly making their way toward the memorial. Erker’s sister-in-law, Margaret Schmidt, recalled him as a man who “loved life, camping and his family.”

Schmidt said that although the family does not come to the ceremony each year, they try to come “on the bigger years” — the fifth anniversary, 10th and this year’s 15th.

At the Pentagon, President Obama paid tribute to those who died in New York, Washington and Pennsylvania during the attacks, and he honored the military’s sacrifices. But his remarks also carried a more pointedly political message directed at Trump, who has called for restrictions on Muslim immigrants.

Obama said the country’s diversity is its strength, and he urged Americans to use Sunday’s anniversary to “reaffirm our character as a nation” and “not to let others divide us.”

“In the end, the most enduring memorial to those we lost is ensuring the America we continue to be,” Obama said. “That we stay true to ourselves. That we stay true to what’s best in us.”

Defense Secretary Ashton Carter preceded Obama’s speech with one that was more bellicose in tone. He warned that terrorists who threaten the United States will “come to feel the righteous fist of American might.”

“Our memory is long, and our reach and resolve is endless,” Carter said.

Meanwhile, in Shanksville, Pa., hundreds gathered to honor the 40 passengers and crew of United Airlines Flight 93 that crashed there. The crash site commemorates the spontaneous actions of those on board who rebelled against the plane’s four hijackers, forcing the aircraft down in the rural valley of the Allegheny mountains in western Pennsylvania.

Firefighters in uniform arrived Sunday to pay homage. So too did United flight attendants who laid roses at the marble wall bearing the names of their seven colleagues.

“Any one of us could have been flying that day,” said Ernie Cornejo, a United flight attendant.

Evidence gathered after the fact concluded that passengers and crew had tried to retake control of the flight — beginning their attack with “Let’s roll,” a mantra that came to embody the nation’s resolve. The terrorists were planning to strike the U.S. Capitol.

The jet was just 20 minutes from Washington when it crashed, leaving the ground smoldering.

Today, the area around the site is a field of wildflowers.

Greg Jaffe contributed to this report.