Video above: Ceremony being held in New York to honor 9/11 victims

Americans are remembering 9/11 with moments of silence, readings of victims' names, volunteer work and other tributes 21 years after the deadliest terror attack on U.S. soil.

A tolling bell and a moment of silence began the commemoration at ground zero in New York, where the World Trade Center's twin towers were destroyed by the hijacked-plane attacks of Sept. 11, 2001. Victims’ relatives and dignitaries also convened at the two other attack sites, the Pentagon and a field in Pennsylvania.

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.

It also stirred — for a time — a sense of national pride and unity for many, while subjecting Muslim Americans to years of suspicion and bigotry and engendering debate over the balance between safety and civil liberties. In ways both subtle and plain, the aftermath of 9/11 ripples through American politics and public life to this day.

Live video: Ceremony held at the Pentagon to honor lives lost on 9/11

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

More than 70 of Sekou Siby's co-workers perished at Windows on the World, the restaurant atop the trade center's north tower. Siby had been scheduled to work that morning until another cook asked him to switch shifts.

Siby never took a restaurant job again; it would have brought back too many memories. The Ivorian immigrant wrestled with how to comprehend such horror in a country where he'd come looking for a better life.

He found it difficult to form the type of close, family-like friendships he and his Windows on the World co-workers had shared. It was too painful, he had learned, to become attached to people when “you have no control over what’s going to happen to them next.”

“Every 9/11 is a reminder of what I lost that I can never recover,” says Siby, who is now president and CEO of ROC United. The restaurant workers' advocacy group evolved from a relief center for Windows on the World workers who lost their jobs when the twin towers fell.

On Sunday, President Joe Biden plans to speak and lay a wreath at the Pentagon, while First Lady Jill Biden is scheduled to speak in Shanksville, Pennsylvania, where one of the hijacked planes went down after passengers and crew members tried to storm the cockpit as the hijackers headed for Washington. Al-Qaida conspirators had seized control of the jets to use them as passenger-filled missiles.

Vice President Kamala Harris and husband Doug Emhoff joined the observance at the National Sept. 11 Memorial in New York, but by tradition, no political figures speak at the ground zero ceremony. It centers instead on victims' relatives reading aloud the names of the dead.

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 which came together — to some extent — after the attacks has 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.

NEW YORK (AP) — Americans remembered 9/11 on Sunday with tear-choked tributes and pleas to “never forget," 21 years after the deadliest terror attack on U.S. soil.

Nikita Shah headed to the ceremony on the ground in a T-shirt that bore the de facto epigraph of the annual commemoration — “never forget” — and the name of her slain father, Jayesh Shah.

The family moved to Houston afterward but has often returned to New York for the anniversary of the attack that killed him and nearly 3,000 other people.

“For us, it was being around people who kind of experienced the same type of grief and the same feelings after 9/11,” said Shah, who was 10 when her father was killed at the World Trade Center on Sept. 11, 2001.

Victims’ relatives and dignitaries also convened at the two other attack sites, the Pentagon and a field in Pennsylvania.

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.

More than two decades later, Sept. 11 remains a point for reflection on the attack that reconfigured national security policy and spurred a U.S. “war on terror” worldwide. Sunday's observances, which follow a fraught milestone anniversary last year, come little more than a month after a U.S. drone strike killed a key al-Qaida figure who helped plot the 9/11 attacks, Ayman al-Zawahri.

It also stirred — for a time — a sense of national pride and unity for many while subjecting Muslim Americans to years of suspicion and bigotry and engendering debate over the balance between safety and civil liberties. In ways both subtle and plain, the aftermath of 9/11 ripples through American politics and public life to this day.

And 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.

Firefighter Jimmy Riches’ namesake nephew wasn’t born yet when his uncle died, but the boy took the podium to pay tribute to him.

“You’re always in my heart. And I know you are watching over me,” he said after reading a portion of the victims’ names.

More than 70 of Sekou Siby's co-workers perished at Windows on the World, the restaurant atop the trade center's north tower. Siby had been scheduled to work that morning until another cook asked him to switch shifts.

Siby never took a restaurant job again; it would have brought back too many memories. The Ivorian immigrant wrestled with how to comprehend such horror in a country where he'd come looking for a better life.

He found it difficult to form the type of close, family-like friendships he and his Windows on the World co-workers had shared. It was too painful, he had learned, to become attached to people when “you have no control over what’s going to happen to them next.”

“Every 9/11 is a reminder of what I lost that I can never recover,” says Siby, who is now president and CEO of ROC United. The restaurant workers' advocacy group evolved from a relief center for Windows on the World workers who lost their jobs when the twin towers fell.

On Sunday, President Joe Biden spoke and laid a wreath at the Pentagon. At the same time, first lady Jill Biden spoke in Shanksville, Pennsylvania, where one of the hijacked planes went down after passengers and crew members tried to storm the cockpit as the hijackers headed for Washington. Al-Qaida conspirators had seized control of the jets to use them as passenger-filled missiles.

Vice President Kamala Harris and husband Doug Emhoff joined the observance at the National Sept. 11 Memorial in New York. Still, by tradition, no political figures speak at the ground zero ceremony. It centers instead on victims' relatives reading aloud the names of the dead.

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.

Three American presidents stood somberly side by side Saturday at the National September 11 Memorial in New York, sharing a moment of silence to mark the anniversary of the nation's worst terrorist attack with a display of unity.

Presidents Joe Biden, Barack Obama and Bill Clinton gathered at the site where the World Trade Center towers fell two decades ago. Each man wore a blue ribbon and held his hands over his heart as a procession marched a flag through the memorial before hundreds of people, some carrying photos of loved ones lost in the attacks.

Before the event began, a jet flew overhead in an eerie echo of the attacks, drawing a glance from Biden toward the sky. For much of the ceremony he stood with his arms crossed and head bowed, listening while the names of the victims were read. At one point, he wiped a tear from his eye.

Biden was a senator when hijackers commandeered four planes and carried out the attack. He was Obama's vice president in 2011 when the country observed the 10th anniversary of the strikes. Saturday's commemoration was his first as commander in chief, beginning in New York City and culminating late afternoon at the Pentagon, where the world's mightiest military suffered an unthinkable blow to its very home.

In between he visited Shanksville, Pennsylvania, where passengers brought down a hijacked plane that was headed for the U.S. Capitol. Biden and his wife, Jill, walked with relatives of the crash victims into the grassy field where the jet came to rest.

He reflected on the need for unity when he dropped by the Shanksville Volunteer Fire Department to deliver Bud Light and thank first responders who responded to the plane crash on Sept. 11.

"Everyone says Biden, 'Why do you keep insisting on trying to bring the country together?'' the president told reporters. "That's the thing that's going to affect our well-being more than anything else."

It is now Biden who shoulders the responsibility borne by his predecessors to prevent another strike. He must do that against fears of a rise in terrorism after the hasty U.S. withdrawal from Afghanistan, where those who planned the Sept. 11 attacks were sheltered.

But on a day when his nation recalled its shock and sorrow, Biden left the speech-making to others.

Biden's vice president, Kamala Harris, spoke in Shanksville at the Flight 93 National Memorial, praising the courage of those passengers and the resilience of Americans who came together in the days after the attacks.

"In a time of outright terror, we turned toward each other," she said. "If we do the hard work of working together as Americans, if we remain united in purpose, we will be prepared for whatever comes next."

Former President George W. Bush, speaking before Harris, recalled how 9/11 showed that Americans could unite despite their differences. It was a message, he said, that was needed today.

"So much of our politics have become a naked appeal to anger, fear and resentment," Bush said. "On America's day of trial and grief, I saw millions of people instinctively grab for a neighbor's hand, and rally for the cause of one another. That is the America I know."

Biden, speaking at the firehouse later, praised Bush's message of unity, and mentioned that he had taken photos with some boys wearing Trump hats at the firehouse. Biden framed the need for unity as a crucial to the success of democracies, asking "Are we going to, in the next four, five, six, 10 years, demonstrate that democracies can work, or not?"

Former President Donald Trump skipped the official 9/11 memorial ceremonies and instead visited a fire station and police precinct in New York.

While Biden had no prepared remarks of his own Saturday, he did offer praise for Bush's words, telling reporters in Pennsylvania that he thought the former president "made a really good speech today. Genuinely."

But unity was a theme that Biden emphasized in a taped address released by the White House late Friday. He spoke about the "true sense of national unity" that emerged after the attacks, seen in "heroism everywhere — in places expected and unexpected."

"To me that's the central lesson of September 11," he said. "Unity is our greatest strength."

Biden is the fourth president to console the nation on the anniversary of that dark day, one that has shaped many of the most consequential domestic and foreign policy decisions made by the chief executives over the past two decades.

Bush was reading a book to Florida schoolchildren when the planes slammed into the World Trade Center. He spent that day being kept out of Washington for security reasons — a decision then-Sen. Biden urged him to reconsider, the current president has written — and then delivered a brief, halting speech that night from the White House to a terrified nation.

The terrorist attack would define Bush's presidency. The following year, he chose Ellis Island as the location to deliver his first anniversary address, the Statue of Liberty over his shoulder as he pledged, "What our enemies have begun, we will finish."

The wars in Iraq and Afghanistan were still deadly when Obama visited the Pentagon to mark his first Sept. 11 in office in 2009.

By the time Obama spoke at the 10th anniversary, attack mastermind Osama bin Laden was dead, killed in a May 2011 Navy SEAL raid. Though the nation remained entangled overseas, and vigilant against terrorist threats, the anniversary became more about healing.

"We reaffirm our commitment to keep a sacred trust with their families — including the children who lost parents, and who have demonstrated such extraordinary resilience. But this anniversary is also about reflecting on what we've learned in the 20 years since that awful morning," Obama said in a statement early Saturday morning.

"That list of lessons is long and growing. But one thing that became clear on 9/11 — and has been clear ever since — is that America has always been home to heroes who run towards danger in order to do what is right."

When they think back on Sept. 11, 2001, Obama said, he and former First Lady Michelle Obama aren't left only with lasting images of two planes flying into the twin towers of the World Trade Center or the wreckage at the other attack sites, but also with the courage of the first responders who acted on that day and in the following weeks and months.

"It's the firefighters running up the stairs as others were running down. The passengers deciding to storm a cockpit, knowing it could be their final act. The volunteers showing up at recruiters' offices across the country in the days that followed, willing to put their lives on the line," the former president wrote.

That same selflessness, Obama said, has been on display "again and again" over the past two decades.