STONYCREEK, Pa. — Jill Biden was worried about her sister.

The first lady, speaking Sunday on the 21st anniversary of 9/11, recalled being concerned on the day of the terrorist attacks that killed nearly 3,000 people about how they would affect her sister, Bonny Jacobs, a longtime flight attendant for United Airlines.

“It’s a job that she’s loved for many years, and I knew that the weight of this tragedy would be heavier for her,” she said. “When I got to her house, I realized that I was right: She hadn’t just lost colleagues — she had lost friends.”

Biden paid tribute Sunday morning at the rural Somerset County memorial to the 40 passengers and crew members of United Flight 93, who fought back against the four hijackers and ultimately prevented another attack when the plane crashed in a field at this site in Stonycreek Township.

Two other hijacked commercial jetliners crashed into the twin towers of the World Trade Center in New York City and a third plane crashed into the Pentagon just outside Washington, D.C. The hijackers aboard Flight 93 were believed to be targeting the U.S. Capitol in Washington.

Various speakers, including family members of those who were on Flight 93 and the volunteers who now help take care of a memorial to those onboard, offered their personal perspective as rain poured down. Rabbi Jeffrey Myers of Pittsburgh’s Tree of Life / Or L’Simcha Congregation spoke briefly during a moment of reflection.

Biden said the “humanity” of those who responded to the terror attacks shined through the “inhumanity” of that day.

Sept. 11 “touched us all. It changed us all,” she said. “But it reminds us that, with courage and kindness, we can be a light in that darkness. It showed us that we are all connected to one another.”

U.S. Interior Secretary Deb Haaland also spoke, saying the actions of those on Flight 93 — who made a sacrifice for generations to come — may be hard for some to fully comprehend.

But Haaland, who is of Native American heritage, said sacrifice in her community is “part of who we are.”

“Everything we do centers on a belief that there are people who have not yet been born but who will benefit from our actions while we are on this earth,” she said. “In times of tragedy and of immense grief, it brings us peace to know that the troubles we experience and the challenges we face add to our collective memory and our experiences as Americans.”

Connie Hasenei, a great-niece of passenger Patricia Cushing, was one of many who were present to honor their late family members.

She said many people she has met only know about the parts of the 9/11 attacks that took place in New York City and outside Washington, D.C., so she has taken it upon herself to teach others about her great-aunt — who was traveling with her sister-in-law to vacation in San Francisco — and her fellow passengers.

“I have a huge responsibility, being part of the younger generation that has a personal connection to not only the 9/11 attacks, but specifically to Flight 93,” she said. “Those 40 were heroes and must never be forgotten.”

After the main event concluded, Biden and union leaders from the Association of Flight Attendants gathered for a more intimate wreath-laying ceremony to honor crew members from all of the flights hijacked on 9/11.

Dressed in their formal uniforms, wings and all, they read the names of each crew member on the four flights. Jeff Heisey, the secretary-treasurer of the AFA’s United Master Executive Council, said the “fabric of our family was forever altered” by the attacks.

“There are no words that will allow us to capture what those who perished that day mean to us,” he said through tears, as Biden stepped to embrace him. “There is no measure of gratitude worthy of their ultimate sacrifice.”