NEW YORK (NewsNation) — Marking the 21st anniversary of the deadliest terrorist attacks on U.S. soil that killed nearly 3,000 people, Americans gathered in remembrance to honor the victims with moments of silence, tear-choked tributes, and pleas to “never forget.”

Victims’ relatives and dignitaries convened Sunday at the places where hijacked jets crashed on Sept. 11, 2001 — the World Trade Center in New York, the Pentagon and Somerset County, Pennsylvania.

President Joe Biden marked the anniversary by taking part in a somber wreath-laying ceremony at the Pentagon, paying tribute to “extraordinary Americans” who gave their lives on one of the nation’s darkest days.

“We will never forget, we will never give up,” Biden said. “Our commitment to preventing another attack on the United States is without end.”

The president was joined by family members of the fallen and first responders who had been at the Pentagon on the day of the attack.

The president tweeted Sunday morning, remembering those who were lost: “Twenty-one years later, we keep alive the memory of all the precious lives stolen from us at Ground Zero, in Shanksville, Pennsylvania, and at the Pentagon. To the families and loved ones who still feel the ache, Jill and I hold you close in our hearts. We will never forget.”

First lady Jill Biden spoke Sunday at the Flight 93 National Memorial Observance in Shanksville, Pennsylvania.

In her remarks, Biden said that after the shock of 9/11 “settled into sorrow” and she had spoken with her husband and children, her thoughts turned to her sister, who continues to work as a flight attendant with United Airlines.

Biden recalled being “scared to death” that her sister Bonny Jacobs was on one of the four hijacked airplanes.

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

Vice President Kamala Harris and her husband attended a commemoration ceremony at the National September 11th Memorial in New York.

The attacks have cast a long shadow on the personal lives of thousands of people who survived, responded or lost loved ones, friends and colleagues.

Readers often add personal remarks that form an alloy of American sentiments about Sept. 11 — grief, anger, toughness, appreciation for first responders and the military, appeals to patriotism, hopes for peace, occasional political barbs, and a poignant accounting of the graduations, weddings, births and daily lives that victims have missed.

Some relatives also lament that a nation that came together — to some extent — after the attacks have since splintered apart. So much so that federal law enforcement and intelligence agencies, which were reshaped to focus on international terrorism after 9/11, now see the threat of domestic violent extremism as equally urgent.

Other communities around the country are marking the day with candlelight vigils, interfaith services and other commemorations. Some Americans are joining in volunteer projects on a day that is federally recognized as both Patriot Day and a National Day of Service and Remembrance.

The observances follow a fraught milestone anniversary last year. It came weeks after the chaotic and humbling end of the Afghanistan war that the U.S. launched in response to the attacks.

But if this Sept. 11 may be less of an inflection point, it remains a point for reflection on the attack that killed nearly 3,000 people, spurred a U.S. “war on terror” worldwide and reconfigured national security policy.

The Associated Press contributed to this report.